[ALSA] sound: ens1370.c fix shadowed variable warning

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