net80211: move references to IF_LLADDR() into ieee80211_freebsd.c

• Move references to IF_LLADDR() into ieee80211_freebsd.c
• Add a comment on one that I need to verify before I move it
• Implement ieee80211_vap_sync_mac_address() which syncs the VAP mac address from the network interface MAC address. This uses FreeBSD-isms (network epoch, IF_LLADDR()) so it shouldn't be in net80211 itself.

Differential Revision: https://reviews.freebsd.org/D50023