sfc: expose FEC stats on Medford2

There's no explicit capability bit, so we just condition them on having
 efx->num_mac_stats >= MC_CMD_MAC_NSTATS_V2.

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