mac80211: implement multi-interface CSA

This implements a fairly simple multi-interface
CSA. It doesn't support multiple channel
contexts so it doesn't support multi-channel.

Once a CSA is started other CSA requests are
denied until the first one is completed. A single
CSA may affect multiple interfaces. CSA can happen
only if all target CSA chandefs are compatible and
all affected interfaces are sharing a single
channel context exclusively.

A new worker is introduced: csa_complete_work
which is used to account per-interface countdowns
and issue the actual channel switch after last
interface completes its CSA.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
(cherry picked from http://mid.gmane.org/1390228033-19162-3-git-send-email-michal.kazior@tieto.com)