Add half/quarter rate 11a channel support:

Add half/quarter rate 11a channel support:
o change handling of regdomain-related mib knobs so they can be set
  post-attach: regdomain, countrycode, outdoor, and xchanmode; the
  hal will not permit changing the regdomain but we expose it for now
o on regdomain/countrycode change recalculate the channel list and
  push it to the net80211 layer (NB: looks to need more tweaking)
o setup rate tables for half/quarter rate channels
o honor half/quarter rate channel configs when changing channels
o honor half/quarter rate channel configs when setting the slot time
o use hack/nonstandard channel numbering scheme for the public safety
  band to avoid overlapping 2.4G channels on dual-band cards
o remove setup of ic_sup_rates; the net80211 layer can do this for us
  and it simplifies handling of half/quarter rate channels

Tested only in Public Safety Band with cards that have RF5112.