Bluetooth: audio: tbs_client: Fix missing cleanup

This fixes repeated attempt to bt_tbs_client_discover that failed
because srv_inst->current_inst was not cleared during previous discovery
process. The issue seen in case the GATT Read CCID returned an error.
The code has been factored out to separate helper function.

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