o add channel flag to enable freq <-> ieee channel # mapping (can
go away in the future when ieee number is precomputed)
o add mapping between 900mhz freq's and channel #'s that gives a
unique channel # for each half/quarter/full width channel
o remove assumptions that half/quarter rate channels on happen in 11a
o remove assumptions that all 11g channels are full width
o ensure ic_curchan is reset on mode change so changing the channel
list (e.g. on countrycode change) doesn't leave curchan set to an
invalid channel
There is still an issue with switching rate sets; to be fixed separately.
MFC after: 1 month
a3a275335d