lib/crypto: sha512: Use underlying functions instead of crypto_simd_usable()

lib/crypto: sha512: Use underlying functions instead of crypto_simd_usable()

Since sha512_kunit tests the fallback code paths without using
crypto_simd_disabled_for_test, make the SHA-512 code just use the
underlying may_use_simd() and irq_fpu_usable() functions directly
instead of crypto_simd_usable().  This eliminates an unnecessary layer.

Link: https://lore.kernel.org/r/20250731223651.136939-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>