hwrng: bcm2835 - Implementation cleanup callback

We should be disabling the RNG in a hwrng::cleanup callback if we are
not longer the system selected RNG, not wait until the device driver is
removed.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>