topology: Fix issue in parsing routes when generating topology binary

We missed parsing the index value, which is used as a use case indicator, when
processing the route objects.

This patch fixes the bug.

Signed-off-by: Fuwei Tang <fuweix.tang@intel.com>
Tested-by: Subhransu S. Prusty <subhransu.s.prusty@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>