Fix error output in CTR selftest

* cipher/cipher-selftest.c (_gcry_selftest_helper_ctr): Change
fprintf(stderr,...) to syslog(); Correct error output for bulk
IV check, plaintext mismatch => ciphertext mismatch.
--

The 'fprintf's were debugging leftover that leaked into commit.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>