net/mlx4_core: Fix register/unreg vlan flow

The reg/unreg vlan code was broken:

1. a wrapped function called another wrapped function, causing a deadlock.

2. unregister_vlan called cmd_box instead of cmd_box_imm, leading to
   incorrectly passed parameters.

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>