firmware: arm_scmi: Quiesce notifications before teardown

In the Linux kernel, the following vulnerability has been resolved:

firmware: arm_scmi: Quiesce notifications before teardown

scmi_notification_exit() clears and releases the notification instance,
but transport callbacks can still deliver incoming notifications until
the TX/RX channels are freed. During remove, an RX interrupt in that
window can enter scmi_notify() while notification state is being torn
down and then dereference freed memory. The same ordering exists on the
probe error path after notification initialization.

The notification late-init worker has a separate lifetime issue: protocol
event registration queues ni->init_work on the system workqueue, so
destroying ni->notify_wq does not drain that work. If the devres group is
released while init_work is still pending or running, the late-init worker
can dereference the freed notification instance.

Quiesce the notification core before TX/RX channels are torn down, then
clean up the channels before releasing the notification core resources.
Use disable_work_sync() so future late-init queueing is rejected and any
already queued or running late-init work has completed before channel
teardown starts.