firmware: arm_scmi: Harden shared memory access in fetch_notification

firmware: arm_scmi: Harden shared memory access in fetch_notification

[ Upstream commit 9bae076cd4e3e3c3dc185cae829d80b2dddec86e ]

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

Fixes: ed045e73d0e7 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>