USB: serial: fix module-license macros

Several GPL-2.0 drivers used "GPL" rather than "GPL v2" in their
MODULE_LICENSE macros; fix the macros to match the licenses.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/omninet.c

@@ -176,4 +176,4 @@ module_usb_serial_driver(serial_drivers, id_table);
 
 MODULE_AUTHOR(DRIVER_AUTHOR);
 MODULE_DESCRIPTION(DRIVER_DESC);
-MODULE_LICENSE("GPL");
+MODULE_LICENSE("GPL v2");