V4L/DVB (12586): Update ALSA capture controls according to selected source.

The patch introduces new snd_saa7134_capsrc_set (code taken from
snd_saa7134_capsrc_put) that updates also the ALSA capture controls during
snd_card_saa7134_capture_prepare and snd_saa7134_capsrc_put.

There can be much more work done in order to unify the control of the card
(now the card's capture source is tuned/switched in saa7134-video.c too), but
I don't have enough time. This work could be a starting point, but it can be
applied as-is too (it doesn't need any further work to make it working).

Signed-off-by: Oldřich Jedlička <oldium.pro@seznam.cz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>