iwlwifi: mvm: set the TX disable bit when doing a chanctx switch

During a channel switch we should tell the firmware to disable TX
temporarily and re-enable it after the switch is done.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>