ALSA: hda/realtek - Allow different pins for shared hp/mic vref check

Add a new field to indicate the possible pin NID for alternative vref
setup for the shared hp/mic.  Although 0x18 is valid for all Realtek
codecs, it'll be different on other vendor's codecs.

Also, drop the sanity check in update_shared_mic_hp() since the
reference pin is set explicitly in the caller side.

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