firmware: arm_scmi: Calculate virtio PDU max size dynamically

SCMI virtio transport maximum PDU size is currently hardcoded at build
time; this will not play well with the possibile retrieval of a different
size at run-time.

Make the virtio transport derive the maximum PDU size from the max_msg_size
provided by the SCMI core.

No functional change.

Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Message-Id: <20241028120151.1301177-3-cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>