* ic_freq is the centre of the primary channel, not the centre of the
HT40/HT80/etc channel. Add a method to access that.
* Add a method to access the centre of the primary channel, including
knowing the centre of the 5/10/20/40/80, versus the primary channel.
Ie, it's the centre of the 40, 80, 160MHz channel.
* Add a method to access the centre frequency of the secondary 80MHz
channel - we don't support VHT yet, but when we do.
* Add methods to access the current channel and the per-dev desired
channel. Ideally drivers that do full offload with a per-vap channel
configuration should use the vap channel, NOT ic_curchan.
Non-offload drivers that require net80211 to change the channel should
be accessing ic_curchan.