Bluetooth: fix shadow warning in hci_disconnect()

use clkoff_cp for hci_cp_read_clock_offset instead of cp
(already defined above).

Suggested-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>