sfc: commonise some MAC configuration code

Refactor it a little as we go, and introduce efx_mcdi_set_mtu() which we
 will later use for ef100 to change MTU without touching other MAC settings.

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