md: Fix checking to use ->disabled instead of ->fips directly.

* cipher/md.c (md_enable): Check by ->disabled.
(_gcry_md_hash_buffer, _gcry_md_hash_buffers_extract): Likewise.

This is the way other implementations (of MAC, CIPHER, PUBKEY) do.
With this change, we will be able to add *_ctl API like CIPHER and
PUBKEY, if needed.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>