net/mlx4_en: Pad ethernet packets smaller than 17 bytes

Hardware can't accept packets smaller than 17 bytes. Therefore need to
pad with zeros.

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