As both the MODULE_LICENSE and the boilerplates are now in sync and clear
that the driver is licensed under the terms of the GPLv2-only, add a
matching SPDX license identifier tag.

Cc: Ralph Metzler <rjkm@metzlerbros.de>
Cc: Manfred Voelkel <mvoelkel@DigitalDevices.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>