hwrng: iproc-rng200 - Move enable/disable in separate function

We are calling the same code for enable and disable the block in various
parts of the driver. Put that code into a new function to reduce code
duplication.

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