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
with vid+pcp).

PR: 230794
Reviewed by: kib@
Approved by: re@ (gjb@)
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D16887