Bluetooth: BAP: Broadcast source fix idle state

The idle state was never set, thus providing a poorly
implemented state machine. Updated the code so that unused
code was removed and so that the idle state is properly set.

Optimized the way that a state is set for all endpoints
for a broadcast source with a new function.

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