ALSA: hda - Fix ADC input-amp handling for Cx20549 codec

It seems that Conexant CX20549 chip handle only a single input-amp even
though the audio-input widget has multiple sources.  This has been never
clear, and I implemented in the current way based on the debug information
I got at the early time -- the device reacts individual input-amp values
for different sources.  This is true for another Conexant codec, but it's
not applied to CX20549 actually.

This patch changes the auto-parser code to handle a single input-amp
per audio-in widget for CX20549.  After applying this, you'll see only a
single "Capture" volume control instead of separate "Mic" or "Line"
captures when the device is set up to use a single ADC.

We haven't tested 20551 and 20561 codecs yet.  If these show the similar
behavior like 20549, they need to set spec->single_adc_amp=1, too.

Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>