Add proper error path (for disabling runtime PM) when registering of
hwrng fails.

Fixes: b329669e ("hwrng: exynos - Add support for Exynos random number generator")
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>