rand32_entropy_device.c: remove atomic_t misuse

There is no reason for storing a pointer into an atomic_t variable here.
Not only because this requires a dubious double cast that breaks on
64-bit builds as atomic_t is a 32-bit type, but also because the comment
in the code already admits that the whole operation isn't atomic anyway
and that it is fine. So let's keep things simple.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>