ALSA: hda - Implement dynamic-ADC switching for VIA codecs

Some VIA codecs like VT1702 provide the input-route only to specific
ADCs such as digital-mic inputs.  These routes aren't covered by the
normal primary ADC, and for now, user had to open the capture stream
assigned to that special ADC manually for using such inputs.

This patch implements a way to switch the current ADC dynamically per
the input-source selection in such a case.  When this workaround is
activated, the driver provides only one capture stream and one input-
source control but with the full possible inputs.  The driver switches
the ADC to be used (or being used) according to the input-source on the
fly.
Signed-off-by: Takashi Iwai <tiwai@suse.de>