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

GPL v2 is the original license according to the old license text.
See f64cdd0e.
Signed-off-by: NMarcus Folkesson <marcus.folkesson@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/usb-skeleton.c

@@ -640,4 +640,4 @@ static struct usb_driver skel_driver = {
 
 module_usb_driver(skel_driver);
 
-MODULE_LICENSE("GPL");
+MODULE_LICENSE("GPL v2");