V4L/DVB (3584): Implement V4L2_TUNER_MODE_LANG1_LANG2 audio mode

V4L/DVB (3584): Implement V4L2_TUNER_MODE_LANG1_LANG2 audio mode

Add a new audio mode V4L2_TUNER_MODE_LANG1_LANG2 (used by VIDIOC_G/S_TUNER).
This mode allows the user to select both languages of a bilingual transmission,
one language on the left, one on the right audio channel. If there is no
bilingual transmission, or it is not supported, then this mode should act like
V4L2_TUNER_MODE_STEREO.
This mode is introduced for PVR-like drivers where it is useful to be able to
record both languages of a bilingual broadcast.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>