sfc: check MTU against minimum threshold

Reported-by: Ma Yuying <yuma@redhat.com>
Suggested-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Reviewed-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>