Bluetooth: L2CAP: Add released callback

This adds a callback to indicate when the stack has released all
references to a given channel so the owner free up any resources
associated with that.

This is requires since EATT channels cannot rely on the destroy callback
as it does not use a fixed channel like ATT.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>