crypto: x86/sha256-avx2 - add missing vzeroupper

Since sha256_transform_rorx() uses ymm registers, execute vzeroupper
before returning from it.  This is necessary to avoid reducing the
performance of SSE code.

Fixes: d34a4600 ("crypto: sha256 - Optimized sha256 x86_64 routine using AVX2's RORX instructions")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>