device: Remove z_device_is_ready

This duplicates the functionality of device_is_ready.

Calls for z_device_is_ready are being done in kernel mode, so it is
safe to call its implementation directly.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>

kernel/device.c

diff --git a/kernel/device.c b/kernel/device.c
index 45806fc..37233f5 100644
--- a/kernel/device.c
+++ b/kernel/device.c
@@ -40,13 +40,13 @@
 	 * performed. Reserve string comparisons for a fallback.
 	 */
 	STRUCT_SECTION_FOREACH(device, dev) {
-		if (z_device_is_ready(dev) && (dev->name == name)) {
+		if (z_impl_device_is_ready(dev) && (dev->name == name)) {
 			return dev;
 		}
 	}
 
 	STRUCT_SECTION_FOREACH(device, dev) {
-		if (z_device_is_ready(dev) && (strcmp(name, dev->name) == 0)) {
+		if (z_impl_device_is_ready(dev) && (strcmp(name, dev->name) == 0)) {
 			return dev;
 		}
 	}
@@ -87,7 +87,7 @@
 	return cnt;
 }
 
-bool z_device_is_ready(const struct device *dev)
+bool z_impl_device_is_ready(const struct device *dev)
 {
 	/*
 	 * if an invalid device pointer is passed as argument, this call