usbip: remove check for negative values for an unsigned value

From: Márton Németh <[email protected]>

The parameter rhport is unsigned so there is no need checking for negative values.

This will remove the following warning message when compiling with "make W=1 ...":
drivers/staging/usbip/vhci_sysfs.c: In function ‘valid_args’:
drivers/staging/usbip/vhci_sysfs.c:138: warning: comparison of unsigned expression < 0 is always false

---

index d9736f9..7b6e4a9 100644
@@ -135,7 +135,7 @@ static DEVICE_ATTR(detach, S_IWUSR, NULL, store_detach);
 static int valid_args(__u32 rhport, enum usb_device_speed speed)
 {
 	/* check rhport */
-	if ((rhport < 0) || (rhport >= VHCI_NPORTS)) {
+	if (rhport >= VHCI_NPORTS) {
 		pr_err("port %u\n", rhport);
 		return -EINVAL;
 	}