lib/crypto: x86/sha512: Remove unnecessary checks for nblocks==0

Since sha512_blocks() is called only with nblocks >= 1, remove
unnecessary checks for nblocks == 0 from the x86 SHA-512 assembly code.

Link: https://lore.kernel.org/r/20250630160320.2888-16-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@kernel.org>