hwrng: amd - release_region must be called after hwrng_unregister

The driver release the memory region before being sure that nobody use
it.

This patch made hwrng_unregister ran before any release was done.

Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>