media: dvb-frontends/stv0910: 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>

drivers/media/dvb-frontends/stv0910.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Driver for the ST STV0910 DVB-S/S2 demodulator.
  *

drivers/media/dvb-frontends/stv0910.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Driver for the ST STV0910 DVB-S/S2 demodulator.
  *

drivers/media/dvb-frontends/stv0910_regs.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * @DVB-S/DVB-S2 STMicroelectronics STV0900 register definitions
  * Author Manfred Voelkel, August 2013