KEYS: use swapped SKID for performing partial matching

Earlier KEYS code used pure subject key identifiers (fingerprint)
for searching keys. Latest merged code removed that and broke
compatibility with integrity subsytem signatures and original
format of module signatures.

This patch returns back partial matching on SKID.

Reported-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: David Howells <dhowells@redhat.com>