iwlwifi: mvm: hold mutex when flushing in iwl_mvm_flush_no_vif()

The iwl_mvm_flush_tx_path() function sends a synchronous command to
the firmware.  When doing that, we must hold the mutex.  The
iwl_mvm_flush_no_vif() function was mistakenly not holding the mutex.
Fix it.

Fixes: 6110d9e5bd ("iwlwifi: mvm: Flush non STA TX queues")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>

drivers/net/wireless/intel/iwlwifi/mvm/mac80211.c

@@ -4005,11 +4005,14 @@ static void iwl_mvm_flush_no_vif(struct iwl_mvm *mvm, u32 queues, bool drop)
 	int i;
 
 	if (!iwl_mvm_has_new_tx_api(mvm)) {
-		if (drop)
+		if (drop) {
+			mutex_lock(&mvm->mutex);
 			iwl_mvm_flush_tx_path(mvm,
 				iwl_mvm_flushable_queues(mvm) & queues, 0);
-		else
+			mutex_unlock(&mvm->mutex);
+		} else {
 			iwl_trans_wait_tx_queues_empty(mvm->trans, queues);
+		}
 		return;
 	}