net: lwip: provide entropy to MBed TLS in one go

net: lwip: provide entropy to MBed TLS in one go

We currently provide entropy to mbedTLS using 8b chunks.
Take into account the 'len' parameter passed by MBed TLS to the entropy
gathering function instead. Note that the current code works because len
is always 128 (defined at compile time), therefore mbedtls_hardware_poll()
is called repeatedly and the buffer is filled correctly. But passing 'len'
to dm_rng_read() is both better and simpler.

Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Suggested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>