Bluetooth: Use hci_disconnect() for mgmt_disconnect_device()

There's no reason to custom build the HCI_Disconnect command in the
Disconnect Device mgmt command handler. This patch updates the code to
use hci_disconnect() instead.

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