8139cp: fix VLAN unregistration

The 8139cp driver did VLAN unregistration incorrectly.
It disables VLAN completely when the first VID is unregistered. It
should instead disable VLAN when the group is unregistered by calling
cp_vlan_rx_register with a NULL grp.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>