hwrng: cctrng - don't open code init and exit functions

Do not open code the init and exit functions of the cctrng driver.

If we move the BUILD_BUG_ON checks into the probe function, we can use
module_platform_driver and make the code shorter.

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