Bluetooth: BAP: Broadcast: Fix state checks

The existing state checks for both the broadcast sink
and broadcast source only ever checked the first BIS.
This sort of made sense, since they are all linked by HCI
(i.e. they share the same state), but there is a race condition
in the ISO and BAP callbacks that could allow applications
to delete sinks and sources before all the ISO callbacks
had been handled.

By treating the sink and source states as the highest value
of the BIS, then we better treat all BIS the same.

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