mac80211: schedule the actual switch of the station before CSA count 0

Due to the time it takes to process the beacon that started the CSA
process, we may be late for the switch if we try to reach exactly
beacon 0.  To avoid that, use count - 1 when calculating the switch time.

Cc: stable@vger.kernel.org
Reported-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>