net/mlx4_en: Fix vlan mask for ethtool steering rules

The vlan mask field should be validated and assigned according to the field
size which is 12 bits. Also replace the numeric 0xfff mask with existing kernel
macro.

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