crypto: x86/sha512-avx2 - add missing vzeroupper

Since sha512_transform_rorx() uses ymm registers, execute vzeroupper
before returning from it.  This is necessary to avoid reducing the
performance of SSE code.

Fixes: e01d69cb ("crypto: sha512 - Optimized SHA512 x86_64 assembly routine using AVX 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>