hwrng: optee - handle unlimited data rates

Data rates of MAX_UINT32 will schedule an unnecessary one jiffy
timeout on the call to msleep. Avoid this scenario by using 0 as the
unlimited data rate.

Signed-off-by: Jorge Ramirez-Ortiz <jorge@foundries.io>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>