ALSA: hda - Fix the check of loopback-mixer element index in patch_via.c

Fix the check of the multiple loopback-mixer, which gave sometimes
a wrong index assigned to an element even for different names, e.g.
Mic and Front Mic.  Now check the label properly for avoid duplication.

Reported-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>