LinuxKPI: 802.11: improve lladdr change to not use ifnet internals

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
Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D47490

(cherry picked from commit edab5a280e0b4a53d0f3e34957075c7efe048e8d)