Bluetooth: CAP: bt_cap_initiator_started called without unicast support fix

The bt_cap_initiator_started may be called in cap_stream_started_cb,
even if there is no unicast support. Added a guard for
CONFIG_BT_BAP_UNICAST to avoid calling an unlinked function.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>