Bluetooth: Track authentication method in SMP context

For Secure Connections we'll select the authentication method as soon as
we receive the public key, but only use it later (both when actually
triggering the method as well as when determining the quality of the
resulting LTK). Store the method therefore in the SMP context.

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