iwlwifi: mvm: clear csa countdown when AP is stopped

The csa_countdown flag was not cleared when the AP is stopped.
As a result, if the AP was stopped after csa_countdown had started,
all the folowing channel switch commands would fail.
Fix that by clearing the csa_countdown flag when the AP is stopped.

Cc: <stable@vger.kernel.org> [3.17+]
Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>