firmware: arm_scmi: Make .clear_channel optional

Make transport operation .clear_channel optional since some transports
do not need it and so avoid to have them implement dummy callbacks.

Link: https://lore.kernel.org/r/20210803131024.40280-7-cristian.marussi@arm.com
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>