if_vlan: Always default to 802.1q.

There is no reason for this fallback to be conditional on COMPAT_FREEBSD12.

PR: 273539
MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Reviewed by: melifaro, allanjude
Differential Revision: https://reviews.freebsd.org/D41717

(cherry picked from commit b451dcc84f1cfd1d14ede8a53d1d8359c9b85c94)