samples: Bluetooth: Handle shutdown of iso broadcast benchmark

The sample did not properly handle ending the broadcast and setting
it up for a new broadcast, due to missing wait for sem_big_term and
a bad check in iso_disconnected.

Furthermore if any did not work when setting up the BIG,
the error handling did not properly clean up for a retry.

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