modbus: 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>

include/modbus/modbus.h

diff --git a/include/modbus/modbus.h b/include/modbus/modbus.h
index 400dc8b..7823494 100644
--- a/include/modbus/modbus.h
+++ b/include/modbus/modbus.h
@@ -487,8 +487,6 @@
  * @param adu        Pointer to the RAW ADU struct
  * @param header     Pointer to the buffer in which MBAP header
  *                   will be placed.
- *
- * @retval           0 If transfer was successful
  */
 void modbus_raw_put_header(const struct modbus_adu *adu, uint8_t *header);
 
@@ -497,8 +495,6 @@
  *
  * @param adu        Pointer to the RAW ADU struct
  * @param header     Pointer to the buffer containing MBAP header
- *
- * @retval           0 If transfer was successful
  */
 void modbus_raw_get_header(struct modbus_adu *adu, const uint8_t *header);