hwrng: iproc-rng200 - do not use static structure

Instead of using static hwrng structure that is reused between
binds/unbinds of the device let's embed it into driver's private
structure that we allocate. This way we are guaranteed not to stumble
onto something left from previous bind attempt.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>