hwrng: timeriomem - Fix potential oops (request_mem_region/__devinit)

Fixed oops when calling device_unregister followed by device_register
(changing __init to __devinit) and removed request_mem_region() as
platform_device_register already does this which can result in EBUSY

Signed-off-by: Alexander Clouter <alex@digriz.org.uk>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>