ALSA: hda/via - Don't create duplicated boost controls

The driver may create duplicated mic boost controls when there are
multiple mics with the very same type / location, and this leads to
the error at actual kcontrol creation.  It needs to check the validity
of the created control and add a proper index if it's duplicated.

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