sfc: determine wire m-port at EF100 PF probe time

Traffic delivered to the (MAE admin) PF could be from either the wire
 or a VF.  The INGRESS_MPORT field of the RX prefix distinguishes these;
 base_mport is the value this field will have for traffic from the wire
 (which should be delivered to the PF's netdevice, not a representor).

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>