ath9k_htc: check for underflow in ath9k_htc_rx_msg()

We check for overflow here, but we don't check for underflow so it
causes a static checker warning.

Fixes: fb9987d0 ('ath9k_htc: Support for AR9271 chipset.')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ath/ath9k/htc_hst.c

@@ -414,7 +414,7 @@ void ath9k_htc_rx_msg(struct htc_target *htc_handle,
 		return;
 	}
 
-	if (epid >= ENDPOINT_MAX) {
+	if (epid < 0 || epid >= ENDPOINT_MAX) {
 		if (pipe_id != USB_REG_IN_PIPE)
 			dev_kfree_skb_any(skb);
 		else