crypto: sahara - fix ahash selftest failure

[ Upstream commit afffcf3db98b9495114b79d5381f8cc3f69476fb ]

update() calls should not modify the result buffer, so add an additional
check for "rctx->last" to make sure that only the final hash value is
copied into the buffer.

Fixes the following selftest failure:
alg: ahash: sahara-sha256 update() used result buffer on test vector 3,
cfg="init+update+final aligned buffer"

Fixes: 5a2bb93f ("crypto: sahara - add support for SHA1/256")
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Sasha Levin <sashal@kernel.org>