ALSA: hda - Set codec->single_adc_amp flag for Realtek codecs

It turned out that Realtek codecs (ALC260, etc) with input amps in
audio-input widgets don't handle the multiple individual input amps.
Thus we need to set codec->single_adc_amp flag for them in general.

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