usb: usbtest: fix a type promotion bug

The "len" here is sometimes negative error codes from
usb_get_descriptor(), so we don't want to type promote them to unsigned
long.

This bug pre-dates the invention of git.
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/506f7935-2cba-41d9-ab5d-ddb6ad6320bd@moroto.mountainSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/misc/usbtest.c

@@ -705,7 +705,7 @@ static int is_good_config(struct usbtest_dev *tdev, int len)
 {
 	struct usb_config_descriptor	*config;
 
-	if (len < sizeof(*config))
+	if (len < (int)sizeof(*config))
 		return 0;
 	config = (struct usb_config_descriptor *) tdev->buf;