Bluetooth: Add L2CAP Disconnect suppport for LE

The normal L2CAP Disconnect request and response are also used for LE
connection oriented channels. Therefore, we can simply use the existing
handler functions for terminating LE based L2CAP channels.

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