Bluetooth: Track changes from HCI Write Simple Pairing Debug Mode command

When the HCI Write Simple Pairing Debug Mode command has been issued,
the result needs to be tracked and stored. The hdev->ssp_debug_mode
variable is already present, but was never updated when the mode in
the controller was actually changed.

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