index is incremented only when AC97_EI_SPDIF and then assigned to
the index field.  Change the temporary name to is_spdif.

sound/pci/ens1370.c:1638:10: warning: symbol 'index' shadows an earlier one
sound/pci/ens1370.c:84:12: originally declared here
Signed-off-by: NHarvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>