Replace unversioned GPL license notice with appropriate SPDX license
identifier, which is GPL 1.0+.

Cc: Andrey Panin <pazke@donpac.ru>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lkml.kernel.org/r/20230517072140.1086660-3-bagasdotme@gmail.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>