Bluetooth: Refactor valid LTK data testing into its own function

This patch refactors valid LTK data testing into its own function. This
will help keep the code readable since there are several tests still
missing that need to be done on the LTK data.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>