mlx4: Fix vlan table overflow

Prevent overflow when trying to register more Vlans then the Vlan table in
HW is configured to.
Need to take into acount that the first 2 entries are reserved.

Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>