firmware: arm_scmi: Use vendor string in max-rx-timeout-ms

The original optional property was missing a vendor string prefix; this
has been rectified.

Fix the naming of such optional property in code too.

Cc: Peng Fan <peng.fan@nxp.com>
Fixes: 1780e411 ("firmware: arm_scmi: Use max-rx-timeout-ms from devicetree")
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Message-Id: <20241028120151.1301177-8-cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>