Unbreak VLANs after r337943.
ether_set_pcp should not be called from ether_output_frame for VLAN
interfaces -- the vid + pcp will be inserted during vlan_transmit in
that case. r337943 sets the VLAN's ifnet's if_pcp to a proper PCP value
and this led to double encapsulation (once with vid 0 and second time