sky2: Fix VLAN unregistration

Fix sky2 disabling VLAN completely when the first vid is unregistered.

sky2 disables VLAN completely when the first VID is unregistered. It
should instead disable VLAN when the group is unregistered by calling
sky2_vlan_rx_register with grp = NULL.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>

 drivers/net/sky2.c |   25 +++++++++++++------------
 1 file changed, 13 insertions(+), 12 deletions(-)
Signed-off-by: Jeff Garzik <jeff@garzik.org>