Bluetooth: Move clock offset reading into hci_disconnect()

To give all hci_disconnect() users the advantage of getting the clock
offset read automatically this patch moves the necessary code from
hci_conn_timeout() into hci_disconnect(). This way we pretty much always
update the clock offset when disconnecting.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>