platform/x86: think-lmi: certificate support clean ups

Complete some clean-ups as reqested from the last review as follow-ups
 - Remove certificate from structure as no need to store it any more
 - Clean up return code handling
 - Moved freeing of signature to before admin object released (issue
   seen in testing when unloading module)
 - Minor code flow improvements

Signed-off-by: Mark Pearson <markpearson@lenovo.com>
Link: https://lore.kernel.org/r/20220321180624.4761-1-markpearson@lenovo.com


Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>