net: iflib: fix vlan processing in the drivers

The logic that sets iri_vtag and M_VLANTAG does not handle the
case where the 802.11q VLAN tag is 0. Fix this issue across
the iflib drivers. While there, also improve and align the
VLAN tag check extraction, by moving it outside the RX descriptor
loop, eliminating a local variable and additional checks.

PR: 260068
Reviewed by: kbowling, gallatin
Reported by: erj
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D33156