Bluetooth: Always confirm incoming SMP just-works requests

For incoming requests we want to let the user know that pairing is
happening since otherwise there could be access to MEDIUM security
services without any user interaction at all. Therefore, set the
selected method to JUST_CFM instead of JUST_WORKS and let it be
converted back to JUST_WORKS later if we are the initators.

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