firmware: arm_scmi: Rename .clear_notification() transport_ops

SCMI transport operation .clear_notification() is indeed a generic method
to clear the channel in a transport dependent way, as such it could be a
useful helper also in other contexts.

Rename such method as .clear_channel(), renaming accordingly also its
already existent call-sites.

No functional change.

Link: https://lore.kernel.org/r/20200420152315.21008-2-cristian.marussi@arm.com


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