hwrng: rockchip - eliminate some unnecessary dereferences

Despite assigning a temporary variable the value of &pdev->dev early on
in the probe function, the probe function then continues to use this
construct when it could just use the local dev variable instead.

Simplify this by using the local dev variable directly.

Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>