Bluetooth: Audio: Stop calling broadcast sink syncable when synced

The syncable callback was called whenever we received the BIGInfo
The BIGInfo is, however, received even while synced (as per the
HCI spec). Add a check to stop calling syncable when synced,
and keep the sink->big value updated based on the ISO states.

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