sys: include: remove @return doc for void functions

For functions returning nothing, there is no need to document
with @return, as Doxgen complains about "documented empty
return type of ...".

Signed-off-by: Daniel Leung <daniel.leung@intel.com>
diff --git a/include/sys/atomic.h b/include/sys/atomic.h
index 9b3eb5d..95d212e 100644
--- a/include/sys/atomic.h
+++ b/include/sys/atomic.h
@@ -182,8 +182,6 @@
  *
  * @param target Address of atomic variable or array.
  * @param bit Bit number (starting from 0).
- *
- * @return N/A
  */
 static inline void atomic_clear_bit(atomic_t *target, int bit)
 {
@@ -200,8 +198,6 @@
  *
  * @param target Address of atomic variable or array.
  * @param bit Bit number (starting from 0).
- *
- * @return N/A
  */
 static inline void atomic_set_bit(atomic_t *target, int bit)
 {
@@ -219,8 +215,6 @@
  * @param target Address of atomic variable or array.
  * @param bit Bit number (starting from 0).
  * @param val true for 1, false for 0.
- *
- * @return N/A
  */
 static inline void atomic_set_bit_to(atomic_t *target, int bit, bool val)
 {
diff --git a/include/sys/dlist.h b/include/sys/dlist.h
index 6f6e1da..50e9f5c 100644
--- a/include/sys/dlist.h
+++ b/include/sys/dlist.h
@@ -192,8 +192,6 @@
  * @brief initialize list to its empty state
  *
  * @param list the doubly-linked list
- *
- * @return N/A
  */
 
 static inline void sys_dlist_init(sys_dlist_t *list)
@@ -208,8 +206,6 @@
  * @brief initialize node to its state when not in a list
  *
  * @param node the node
- *
- * @return N/A
  */
 
 static inline void sys_dnode_init(sys_dnode_t *node)
@@ -403,8 +399,6 @@
  *
  * @param list the doubly-linked list to operate on
  * @param node the element to append
- *
- * @return N/A
  */
 
 static inline void sys_dlist_append(sys_dlist_t *list, sys_dnode_t *node)
@@ -425,8 +419,6 @@
  *
  * @param list the doubly-linked list to operate on
  * @param node the element to append
- *
- * @return N/A
  */
 
 static inline void sys_dlist_prepend(sys_dlist_t *list, sys_dnode_t *node)
@@ -471,8 +463,6 @@
  * @param cond a function that determines if the current node is the correct
  *             insert point
  * @param data parameter to cond()
- *
- * @return N/A
  */
 
 static inline void sys_dlist_insert_at(sys_dlist_t *list, sys_dnode_t *node,
@@ -501,8 +491,6 @@
  * This and other sys_dlist_*() functions are not thread safe.
  *
  * @param node the node to remove
- *
- * @return N/A
  */
 
 static inline void sys_dlist_remove(sys_dnode_t *node)
diff --git a/include/sys/mutex.h b/include/sys/mutex.h
index 7a3d335..71aec45 100644
--- a/include/sys/mutex.h
+++ b/include/sys/mutex.h
@@ -64,8 +64,6 @@
  * and the mutex was not created with SYS_MUTEX_DEFINE().
  *
  * @param mutex Address of the mutex.
- *
- * @return N/A
  */
 static inline void sys_mutex_init(struct sys_mutex *mutex)
 {
diff --git a/include/sys/printk.h b/include/sys/printk.h
index a652f66..0a6d49e 100644
--- a/include/sys/printk.h
+++ b/include/sys/printk.h
@@ -44,8 +44,6 @@
  *
  * @param fmt Format string.
  * @param ... Optional list of format arguments.
- *
- * @return N/A
  */
 #ifdef CONFIG_PRINTK
 
diff --git a/include/sys/reboot.h b/include/sys/reboot.h
index 59384b9..5652fc5 100644
--- a/include/sys/reboot.h
+++ b/include/sys/reboot.h
@@ -31,10 +31,7 @@
  * SYS_REBOOT_WARM).
  *
  * When successful, this routine does not return.
- *
- * @return N/A
  */
-
 extern FUNC_NORETURN void sys_reboot(int type);
 
 #ifdef __cplusplus