firmware: arm_scmi: Fix kernel doc warnings about return values

Kernel doc validation script still complains about the following:

|No description found for return value of 'scmi_get_protocol_device'
|No description found for return value of 'scmi_devm_notifier_register'
|No description found for return value of 'scmi_devm_notifier_unregister'

Fix adding missing Return kernel-doc statements.

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Cristian Marussi <[e-mail]>
Signed-off-by: Sudeep Holla <[e-mail]>

drivers/firmware/arm_scmi/driver.c

···
  * @proto_id and @name: if device was still not existent it is created as a
  * child of the specified SCMI instance @info and its transport properly
  * initialized as usual.
+ *
+ * Return: A properly initialized scmi device, NULL otherwise.
  */
 static inline struct scmi_device *
 scmi_get_protocol_device(struct device_node *np, struct scmi_info *info,

drivers/firmware/arm_scmi/notify.c

···
  *
  * Generic devres managed helper to register a notifier_block against a
  * protocol event.
+ *
+ * Return: 0 on Success
  */
 static int scmi_devm_notifier_register(struct scmi_device *sdev,
 				       u8 proto_id, u8 evt_id,
···
  * Generic devres managed helper to explicitly un-register a notifier_block
  * against a protocol event, which was previously registered using the above
  * @scmi_devm_notifier_register.
+ *
+ * Return: 0 on Success
  */
 static int scmi_devm_notifier_unregister(struct scmi_device *sdev,
 					 u8 proto_id, u8 evt_id,