Re-add a status check which sneaked out during r214804.

This change can fix some USB error messages showing up
during bootup.

MFC after:	3 days
Approved by:	thompsa (mentor)

sys/dev/usb/usb_request.c

@@ -793,6 +793,10 @@ usbd_req_reset_port(struct usb_device *udev, struct mtx *mtx, uint8_t port)
 		if (err) {
 			goto done;
 		}
+		/* if the device disappeared, just give up */
+		if (!(UGETW(ps.wPortStatus) & UPS_CURRENT_CONNECT_STATUS)) {
+			goto done;
+		}
 		/* check if reset is complete */
 		if (UGETW(ps.wPortChange) & UPS_C_PORT_RESET) {
 			break;