Bluetooth: audio: tbs: Make bt_tbs_client_call to contain URI pointer

To avoid compilation issues when including tbs.h header file when
CONFIG_BT_TBS_MAX_URI_LENGTH is not set, just pass the URI pointer
instead of an array in bt_tbs_client_call structure.

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