Call scmi_notification_exit() only when SCMI platform driver instance has
been really successfully removed.

Link: https://lore.kernel.org/r/20210112191326.29091-1-cristian.marussi@arm.com
Fixes: 6b8a6913 ("firmware: arm_scmi: Enable notification core")
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
[sudeep.holla: Move the call outside the list mutex locking]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>