firmware: arm_scmi: Reject clear channel request on A2P

The clear channel transport operation is supposed to be called exclusively
on the P2A channel from the agent, since it relinquishes the ownership of
the channel to the platform, after this latter has initiated some sort of
P2A communication.

Make sure that, if it is ever called on a A2P, is logged and ignored.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Message-Id: <20241021171544.2579551-1-cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>