[Bluetooth] Track status of Simple Pairing mode

The Simple Pairing feature is optional and needs to be enabled by the
host stack first. The Linux kernel relies on the Bluetooth daemon to
either enable or disable it, but at any time it needs to know the
current state of the Simple Pairing mode. So track any changes made
by external entities and store the current mode in the HCI device
structure.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>