media: dvb-frontends/mxl5xx: add SPDX license identifier

As it is clear that the driver is licensed under the terms of GPLv2-only
by now, add a matching SPDX license identifier to all driver files.

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>