Add initial support for 900MHz cards like the Ubiquiti SR9:

Add initial support for 900MHz cards like the Ubiquiti SR9:
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