Bluetooth: Add support for setting SSP debug mode

Enabling and disabling SSP debug mode is useful for development. This
adds a debugfs entry that allows to configure the SSP debug mode.

On purpose this has been implemented as debugfs entry and not a public
API since it is really only useful during testing and development.

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