hwrng: iproc-rng200 - Fix disable of the block.

When trying to disable the block we bitwise or the control
register with value zero. This is confusing as using bitwise or with
value zero doesn't have any effect at all. Drop this as we already set
the enable bit to zero by appling inverted RNG_RBGEN_MASK.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Acked-by: Scott Branden <scott.branden@broadcom.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>