iwlwifi: fix queue flush confusion

The flush_control parameter to iwlagn_txfifo_flush
is passed as an internal value (context flags) and
then sent to the device, that can't be right.

Fix the confusion by removing the parameter, always
use IWL_DROP_ALL that is redefined according to the
firmware API in the flush control.

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