Bluetooth: Add support for setting DUT mode

The Device Under Test (DUT) mode is useful for doing certification
testing and so expose this as debugfs option.

This mode is actually special since you can only enter it. Restoring
normal operation means that a HCI Reset is required. The current mode
value gets tracked as a new device flag and when disabling it, the
correct command to reset the controller is sent.

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