net/mlx4_en: loopbacked packets are dropped when SMAC=DMAC

Should NOT check SMAC=DMAC when:
1. loopback is turned on
2. validate_loopback is true.

Fixed it accordingly.

Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>