[media] v4l2: rename V4L2_TUNER_ADC to V4L2_TUNER_SDR

[media] v4l2: rename V4L2_TUNER_ADC to V4L2_TUNER_SDR

SDR receiver has ADC (Analog-to-Digital Converter) and SDR transmitter
has DAC (Digital-to-Analog Converter). Originally I though it could
be good idea to have own type for receiver and transmitter, but now I
feel one common type for SDR is enough. So lets rename it.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Acked-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
[hans.verkuil@cisco.com: this was added in 4.4, so update 4.2 to 4.4]

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>