usb: usb-skeleton: make MODULE_LICENSE and SPDX tag match

GPL v2 is the original license according to the old license text. See f64cdd0e94f1faf3b7f2b03af71f70dc6d8da0c2.

---
 drivers/usb/usb-skeleton.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 26ca0ec01fd5..c3ddd0f1f449 100644
@@ -640,4 +640,4 @@ static struct usb_driver skel_driver = {
 
 module_usb_driver(skel_driver);
 
-MODULE_LICENSE("GPL");
+MODULE_LICENSE("GPL v2");