x86/microcode/AMD: Use sha256() instead of init/update/final

Just call sha256() instead of doing the init/update/final sequence.

No functional changes.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250428183006.782501-1-ebiggers@kernel.org