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