vlan: Don't check for vlan group before vlan_tx_tag_present.

Many (but not all) drivers check to see whether there is a vlan
group configured before using a tag stored in the skb.  There's
not much point in this check since it just throws away data that
should only be present in the expected circumstances.  However,
it will soon be legal and expected to get a vlan tag when no
vlan group is configured, so remove this check from all drivers
to avoid dropping the tags.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>