firmware: arm_scmi: Harden shared memory access in fetch_notification

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

Fixes: d5141f37

 ("firmware: arm_scmi: Add notifications support in transport layer")
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20221222183823.518856-4-cristian.marussi@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>