[ALSA] sound: virtuoso.c fix shadowed variable warning

Use priv_idx as an identifier.
sound/pci/oxygen/virtuoso.c:277:15: warning: symbol 'index' shadows an earlier one
sound/pci/oxygen/virtuoso.c:56:12: originally declared here

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>