Commit 7c1da8d0 "crypto: sha - SHA1 transform x86_64 AVX2"
accidentally disabled the AVX variant by making the avx_usable() test
not only fail in case the CPU doesn't support AVX or OSXSAVE but also
if it doesn't support AVX2.

Fix that regression by splitting up the AVX/AVX2 test into two
functions. Also test for the BMI1 extension in the avx2_usable() test
as the AVX2 implementation not only makes use of BMI2 but also BMI1
instructions.

Cc: Chandramouli Narayanan <mouli@linux.intel.com>
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Reviewed-by: H. Peter Anvin <hpa@linux.intel.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>