ALSA: hda/realtek - Clean up some spec fields

Remove some fields from struct alc_spec, and clean up the usage.
Namely,
- spec->input_mux becomes a single element, private_imux[] is removed
- spec->adc_nids becomes an array by itself, and private_adc_nids[]
  gets removed, too

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