ALSA: hda/realtek - Fix ADC assignment with a shared HP/Mic pin

The recent Realtek driver tries to assign an extra input via the
headphone plug when only a single input source is found.  The code
worked on Samsung Q1, but it broke ASUS 1015 where the mic is a
digital-mic and only a specific ADC works.

This patch fixes the assignment of ADC in the shared mic/hp case.
Instead of assuming the single ADC at first, reduce the ADCs after
trying to assign both mic and HP pins.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=42973



Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>