ALSA: hda - Fix initialization of secondary capture source on VT1705

VT1705 codec has two ADCs where the secondary ADC has no MUX but only
a fixed connection to the mic pin.  This confused the driver and it
tries always overriding the input-source selection by assumption of
the existing MUX for the secondary ADC, resulted in resetting the
input-source at each time PM (including power-saving) occurs.

The fix is simply to check the existence of MUX for secondary ADCs in
the initialization code.

Tested-by: Anisse Astier <anisse@astier.eu>
Cc: <stable@kernel.org> [v3.1+]
Signed-off-by: Takashi Iwai <tiwai@suse.de>