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

For some reason the VLAN code insists on the driver providing a
vlan_rx_kill_vid function even if only NETIF_F_HW_VLAN_RX and not
NETIF_F_HW_VLAN_FILTER is set, so this patch keeps an empty
function. This seems to be a bug though, vlan_rx_add_vid is only
required with NETIF_F_HW_VLAN_FILTER. Ben?