Bluetooth: Fix returning proper cmd_complete for mgmt_disconnect

The Disconnect Management command should return Command Complete instead
of Command Status whenever possible so that user space can distinguish
exactly which command failed in the case of multiple commands. This
patch does the necessary changes in the disconnect command handler to
return the right event to user space.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>