bridge: fix lookup for untagged packets in bridge_transmit()

b0e38a1373 improved if_bridge's ability to cope with different VLANs,
but it failed to update bridge_transmit() to cope with the new rule that
untagged packets are treated as having VLAN ID 0 (rather than 1, as used
to be the case).

Fix that oversight.

PR: 270559
Reviewed by: kp