The changes from 4aff4048 were based on net80211 which is still using
if_private.h details. Adjust the code to no longer depend on internal
details of ifnet. We also switch the "check"-functionn from if_init to
if_transmit which we can query. Given we do have a per-vif eventhandler
we could do without that check but would still need to check
if_getflags() for IFF_UP.
Sponsored by: The FreeBSD Foundation
Reported by: jhibbits
Fixes: 4aff4048
MFC after: 3 days