lib/crypto: sha256: Use underlying functions instead of crypto_simd_usable()

lib/crypto: sha256: Use underlying functions instead of crypto_simd_usable()

Since sha256_kunit tests the fallback code paths without using
crypto_simd_disabled_for_test, make the SHA-256 code just use the
underlying may_use_simd() and irq_fpu_usable() functions directly
instead of crypto_simd_usable().  This eliminates an unnecessary layer.

While doing this, also add likely() annotations, and fix a minor
inconsistency where the static keys in the sha256.h files were in a
different place than in the corresponding sha1.h and sha512.h files.

Link: https://lore.kernel.org/r/20250731223510.136650-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>