firmware: arm_scmi: Harden shared memory access in fetch_response

A misbheaving SCMI platform firmware could reply with out-of-spec messages,
shorter than the mimimum size comprising a header and a status field.

Harden shmem_fetch_response to properly truncate such a bad messages.

Fixes: 5c8a47a5

 ("firmware: arm_scmi: Make scmi core independent of the transport type")
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20221222183823.518856-3-cristian.marussi@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>