Bluetooth: audio: fix bug in scan delegator sync

In the scan delegator when modifying the source we only want to
send a sync request to the upper layers if we are not yet synced,
and not also on a state change

Note that without this change the babblesim test for long writes
will fail

Signed-off-by: Andries Kruithof <andries.kruithof@nordicsemi.no>

Scan del bugfix

Signed-off-by: Andries Kruithof <andries.kruithof@nordicsemi.no>