mac80211: don't filter out beacons once we start CSA

I hit a bug in which we started a CSA with an action frame,
but the AP changed its mind and didn't change the beacon.
The CSA wasn't cancelled and we lost the connection.

The beacons were ignored because they never changed: they
never contained any CSA IE. Because they never changed, the
CRC of the beacon didn't change either which made us ignore
the beacons instead of processing them.

Now what happens is:
1) beacon has CRC X and it is valid. No CSA IE in the beacon
2) as long as beacon's CRC X, don't process their IEs
3) rx action frame with CSA
4) invalidate the beacon's CRC
5) rx beacon, CRC is still X, but now it is invalid
6) process the beacon, detect there is no CSA IE
7) abort CSA

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20201206145305.83470b8407e6.I739b907598001362744692744be15335436b8351@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>