hwrng: cctrng - let devres enable the clock

Call devm_clk_get_optional_enabled in the cctrng driver. We don't have to
disable and unprepare the clock any more in error paths or in the remove
function.

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