hwrng: rockchip - rst is used only during probe

The driver uses the rst variable only for an initial reset when the chip
is probed. There's no need to store rst in the driver's private data, we
can make it a local variable in the probe function.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>