Add sha224 implementation to sha256_ssse3 module.

This also fixes sha256_ssse3 module autoloading issue when 'sha224' is used
before 'sha256'. Previously in such case, just sha256_generic was loaded and
not sha256_ssse3 (since it did not provide sha224). Now if 'sha256' was used
after 'sha224' usage, sha256_ssse3 would remain unloaded.

Cc: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>