Bluetooth: audio: tbs_client: Fix GTBS index returned in API callbacks

This fixes GTBS index used in API. The index that is exposed to the
application is 255 in case of GTBS.
The code has been reworked to remove redundant booleans and index that
was kept inside instance.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>