ALSA: hda - Don't create duplicated ctls for loopback paths

commit 43a8e50a46a4e1dd1451e4a4ffa1f7695fb7d287 upstream.

AD1986A mic pins (0x1d and 0x1f) share the same widget for controlling
the loopback volume/mute, but the generic parser didn't check it.
This ended up with the duplicated controls for the same effect.

This patch adds the check of the duplication for avoiding it.

After this fix, there will be only one control although it affects
both paths; this remaining issue should be fixed later in a different
patch.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=66621


Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>