crypto: sha256 - use crypto_[un]register_shashes

Combine all shash algs to be registered and use new crypto_[un]register_shashes
functions. This simplifies init/exit code.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>