[net80211] prepare for 11ac aware NICs that want to know per-vdev channel and centre frequencies.

• 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.