ALSA: hda - Unify input label creations in generic parser

There are a few places creating the labels and indices of kctls for
each input pin in the current generic parser code.  This is redundant
and makes harder to maintain.  Let's create the labels and indices at
once and keep them in hda_gen_spec.

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