hwrng: omap - Switch to non-obsolete read API implementation

The ".data_present" and ".data_read" operations are marked as OBSOLETE
in the hwrng API. We have to use the ".read" operation instead. It makes
the driver simpler and moves the busy loop, that waits until enough data
is generated, to the read function. We simplify this step by only
checking the status of the engine, if there is data, we copy the data to
the output buffer and the amout of copied data is returned to the caller,
otherwise zero is returned. The hwrng core will re-call the read operation
as many times as required until enough data has been copied.

Signed-off-by: Romain Perier <romain.perier@free-electrons.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>