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/20210712143504.33541-1-cristian.marussi@arm.com
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>