[ALSA] sound: ice1712.c fix shadowed variable warnings

In all four case, adding a private value to the iooff index,
call it priv_idx.
sound/pci/ice1712/ice1712.c:1300:6: warning: symbol 'index' shadows an earlier one
sound/pci/ice1712/ice1712.c:85:12: originally declared here
sound/pci/ice1712/ice1712.c:1312:6: warning: symbol 'index' shadows an earlier one
sound/pci/ice1712/ice1712.c:85:12: originally declared here
sound/pci/ice1712/ice1712.c:1338:6: warning: symbol 'index' shadows an earlier one
sound/pci/ice1712/ice1712.c:85:12: originally declared here
sound/pci/ice1712/ice1712.c:1350:6: warning: symbol 'index' shadows an earlier one
sound/pci/ice1712/ice1712.c:85:12: originally declared here

[tiwai - fixed coding issues as well]

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