cfg80211: add channel tracking for AP and mesh

We need to know which channel is used by a running
AP and mesh for channel context accounting and
finding matching/active interface combination.

STA/IBSS have current_bss already which allows us
to check which channel a vif is tuned to.
Non-fixed channel IBSS can be handled with
additional changes.

Monitor mode is going to be handled differently.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>