hwrng: n2 - limit error spewage when self-test fails

If the self-test fails, it probably won't actually suddenly
start working.  Currently, this causes an endless spew of
error messages on the console and in the logs, so this patch
adds a limiter to the test.

Reported-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>