hwrng: cn10k - Make check_rng_health() return an error code

Currently check_rng_health() returns zero unconditionally.
Make it to output an error code and return it.

Fixes: 38e9791a

 ("hwrng: cn10k - Add random number generator support")
Signed-off-by: Vladis Dronov <vdronov@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>