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

Signed-off-by: Torsten Duwe <[e-mail]>
Signed-off-by: Theodore Ts'o <[e-mail]>

drivers/char/hw_random/core.c

···
 		add_hwgenerator_randomness((void *)rng_fillbuf, rc,
 					   (rc*current_quality)>>10);
 	}
-	hwrng_fill = 0;
+	hwrng_fill = NULL;
 	return 0;
 }