KEYS: Fix public_key asymmetric key subtype name

The length of the name of an asymmetric key subtype must be stored in struct
asymmetric_key_subtype::name_len so that it can be matched by a search for
"<subkey_name>:<partial_fingerprint>".  Fix the public_key subtype to have
name_len set.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>