efi: use sha256() instead of open coding

Now that there's a function that calculates the SHA-256 digest of a
buffer in one step, use it instead of sha256_init() + sha256_update() +
sha256_final().

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Cc: linux-efi@vger.kernel.org
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>