virtio: rng: Check length before copying

Check the length of data written by the device is consistent with the
size of the buffers to avoid out-of-bounds memory accesses in case
values aren't consistent.

Signed-off-by: Andrew Scull <ascull@google.com>
Cc: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>