ALSA: hda - Fix conflicting volume controls on ALC260

ALC260 auto-parsing mode may create multiple controls for the same volume
widget (0x08 and 0x09) depending on the pin.  For example, Front and
Headphone volumes may control the same volume, just the latter one wins.

This patch adds a proper check of the existing of the volume control
and avoid the doulbed creation of the same volume controls.

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