Mark the imx_rngc_probe function as __init.

There's no need to support hotplugging in the imx-rngc driver. We use
module_platform_driver_probe, the probe function will only be called at
startup.
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>