net80211/drivers: improve ieee80211_rx_stats for band

While IEEE80211_R_BAND was defined, there was no place to store the
band. Add a field for that, adjust ieee80211_lookup_channel_rxstatus()
to require it, and update drivers passing "R_{FREQ|IEEE}" in already to
provide the band as well. For the moment keep the fall-back code
requiring all three fields.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D30662