hwrng: tx4939 - Use devm_hwrng_register instead of hwrng_register

By using devm_hwrng_register instead of hwrng_register the .remove
callback in platform_driver can be removed. This reduces a few lines in
code.

Signed-off-by: PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>