net/mlx4_en: Use vlan id instead of vlan index for unregistration

Use of vlan_index created problems unregistering vlans on guests.

In addition, tools delete vlan by tag, not by index, lets follow that.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>