hwrng: amd - Convert to new hwrng read() API

This patch convert the hwrng interface used by amd768-rng to its new API
by replacing data_read()/data_present() by read().

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