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

In both cases we are passing around the substream number, use
sub_num for this.
sound/pci/riptide/riptide.c:1633:6: warning: symbol 'index' shadows an earlier one
sound/pci/riptide/riptide.c:121:12: originally declared here
sound/pci/riptide/riptide.c:1673:6: warning: symbol 'index' shadows an earlier one
sound/pci/riptide/riptide.c:121:12: originally declared here

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