ALSA: hda - Fix capture source selection in patch_via.c

The fixed widget NIDs in patch_via.c seem wrong for some codecs,
and it resulted in the invalid capture source selection.

This patch adds the code to parse the topology instead of using
fixed numbers in order to get the right MUX widget id corresponding
to the ADCs.

Signed-off-by: Takashi Iwai <tiwai@suse.de>