iwlagn: signedness bug

re-apply the unsigned shorts bug fixed by Dan Carpenter but get lost
after the file move.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/iwl-trans-pcie-tx.c

@@ -544,7 +544,7 @@ int iwl_trans_pcie_tx_agg_alloc(struct iwl_trans *trans,
 	struct iwl_trans_pcie *trans_pcie = IWL_TRANS_GET_PCIE_TRANS(trans);
 	struct iwl_tid_data *tid_data;
 	unsigned long flags;
-	u16 txq_id;
+	int txq_id;
 
 	txq_id = iwlagn_txq_ctx_activate_free(trans);
 	if (txq_id == -1) {