crypto: kdf - silence noisy self-test

Make the kdf_sp800108 self-test only print a message on success when
fips_enabled, so that it's consistent with testmgr.c and doesn't spam
the kernel log with a message that isn't really important.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>