Bluetooth: Classic: HFP_AG: Register SCO connect callback

There is an issue that the SCO connect cannot be un-referenced by HFP
AG when the SCO connection is broken if the SCO connect is not created
by HFP AG.

Register SCO connect change callback. And un-reference the SCO connect
in SCO disconnected callback.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>