[media] v4l: add enum_freq_bands support to tuner sub-device

Add VIDIOC_ENUM_FREQ_BANDS, enumerate supported frequency bands,
IOCTL support for sub-device tuners too.

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>