net/mlx4_en: Change loopback only upon feature change

Currently any change of netdev features results in a call to
mlx4_en_update_loopback_state(). Those calls are unnecessary,
and should be called only upon loopback feature change.

Also moved some of the logic into mlx4_en_update_loopback_state().

Signed-off-by: Ido Shamay <idos@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>