Bluetooth: BAP: Fix bt_bap_scan_delegator_find_state

The bt_bap_scan_delegator_find_state did not properly return the
correct receive state due to confusing return value of the
iterator function.

Modify it to use a simple bool.

This fixes all issues with "Failed to find receive state for sink"
when using the Broadcast Sink.

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