ALSA: hda/realtek - Allow multiple individual capture volume/switch controls

So far we create only "Capture Volume" and "Capture Switch" controls
for binding all possible amps, but we'd prefer creating individual
capture volume and switch controls per input in some cases
(e.g. conexant parser does it).

Add a new flag, spec->multi_cap_vol, to follow that policy.

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