lib/crypto: sha256: Sync sha256_update() with sha512_update()

lib/crypto: sha256: Sync sha256_update() with sha512_update()

The BLOCK_HASH_UPDATE_BLOCKS macro is difficult to read.  For now, let's
just write the update explicitly in the straightforward way, mirroring
sha512_update().  It's possible that we'll bring back a macro for this
later, but it needs to be properly justified and hopefully a bit more
readable.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250630160645.3198-14-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>