Samples: Bluetooth: Fix scanning restart for broadcast audio sink

If waiting on the first semaphore in the main loop times out, the main
loop starts from the beginning and attempts to start scanning for
broadcast sources. But since the first semaphore was not passed, the
scan_recv_cb callback was not called yet and thus the previously started
scanning was not stopped. As a result, an attempt to start the scanning
will fail with -EALREADY and the main thread will end. To solve this,
-EALREADY is now ignored when checking the return value of
bt_audio_broadcast_sink_scan_start.

Signed-off-by: Ladislav Podivin <ladislav.podivin@tietoevry.com>