mac80211: use csa counter offsets instead of csa_active

vif->csa_active is protected by mutexes only. This
means it is unreliable to depend on it on codeflow
in non-sleepable beacon and CSA code. There was no
guarantee to have vif->csa_active update be
visible before beacons are updated on SMP systems.

Using csa counter offsets which are embedded in
beacon struct (and thus are protected with single
RCU assignment) is much safer.

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