bna: fix vlan tag stripping and implement its toggling

The recent commit "fe1624cf bna: RX Filter Enhancements" disables
VLAN tag stripping if the NIC is in promiscuous mode. This causes
__vlan_hwaccel_put_tag() is called when the stripping is disabled.
Because of this VLAN over bna does not work and causes BUGs in conjunction
with openvswitch like this:
Reviewed-by: Jiri Pirko <jiri@resnulli.us>

Signed-off-by: David S. Miller <davem@davemloft.net>