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.
Signed-off-by: Cristian Marussi <cristian.marussi@xxxxxxx>