crypto: sha256_ssse3 - also test for BMI2

The AVX2 implementation also uses BMI2 instructions,
but doesn't test for their availability. The assumption
that AVX2 and BMI2 always go together is false. Some
Haswells have AVX2 but not BMI2.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>