net/mlx4_en: Add RX-FCS support

Enabled when device supports KEEP FCS. When the flag is set, Ethernet FCS
is appended to the end of the frame, controlled by ethtool.

Signed-off-by: Muhammad Mahajna <muhammadm@mellanox.com>
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>