be2net: fix to limit max vlans supported in certain skews

In certain skews the ASIC can support only 16 vlans per interface.
Once the limit is crossed, the ASIC is programmed in vlan promiscuous mode.
Switch off the vlan promiscuous mode once the number of vlans
falls back to the max vlans supported.

Signed-off-by: Ajit Khaparde <ajitk@serverengines.com>
Signed-off-by: David S. Miller <davem@davemloft.net>