Bluetooth: audio: tbs_client: Fix missing discovery complete event

This fixes missing Fix bt_tbs_client_cb.discover call that has been
observed when CONFIG_BT_TBS_CLIENT_CCID was disabled. In such case the
discovery proces was not finished from application point of view,
as the callback was not called.

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