sfc: determine representee m-port for EF100 representors

An MAE port, or m-port, is a port (source/destination for traffic) on
 the Match-Action Engine (the internal switch on EF100).
Representors will use their representee's m-port for two purposes: as
 a destination override on TX from the representor, and as a source
 match in 'default rules' to steer representee traffic (when not
 matched by e.g. a TC flower rule) to representor RX via the parent
 PF's receive queue.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>