hw_random: fix sparse warning (NULL vs 0 for pointer)

Signed-off-by: NTorsten Duwe <duwe@suse.de>
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>

drivers/char/hw_random/core.c

@@ -344,7 +344,7 @@ static int hwrng_fillfn(void *unused)
 		add_hwgenerator_randomness((void *)rng_fillbuf, rc,
 					   (rc*current_quality)>>10);
 	}
-	hwrng_fill = 0;
+	hwrng_fill = NULL;
 	return 0;
 }