thunderbolt: Use NULL instead of 0 in nhi.c

'descriptors' is a pointer. Use NULL isntead of 0.
Signed-off-by: Sachin Kamat <sachin.kamat@samsung.com>
Acked-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/thunderbolt/nhi.c

@@ -416,7 +416,7 @@ void ring_free(struct tb_ring *ring)
 			  ring->size * sizeof(*ring->descriptors),
 			  ring->descriptors, ring->descriptors_dma);
 
-	ring->descriptors = 0;
+	ring->descriptors = NULL;
 	ring->descriptors_dma = 0;