r8152: check budget for r8152_poll()

According to the document of napi, there is no rx process when the
budget is 0. Therefore, r8152_poll() has to return 0 directly when the
budget is equal to 0.

Fixes: d2187f8e ("r8152: divide the tx and rx bottom functions")
Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/r8152.c

@@ -2636,6 +2636,9 @@ static int r8152_poll(struct napi_struct *napi, int budget)
 	struct r8152 *tp = container_of(napi, struct r8152, napi);
 	int work_done;
 
+	if (!budget)
+		return 0;
+
 	work_done = rx_bottom(tp, budget);
 
 	if (work_done < budget) {