Bluetooth: Make hci_disconnect() behave correctly for all states

There are a few places that don't explicitly check the connection
state before calling hci_disconnect(). To make this API do the right
thing take advantage of the new hci_abort_conn() API and also make
sure to only read the clock offset if we're really connected.

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