staging: comedi: mite: (!foo) preferred over (foo == NULL)

Also, clarify the 'ring' allocation failure by returning NULL instead
of 'ring' (which would be NULL).
Signed-off-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: NIan Abbott <abbotti@mev.co.uk>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/mite.c

@@ -186,10 +186,10 @@ struct mite_dma_descriptor_ring *mite_alloc_ring(struct mite_struct *mite)
 	struct mite_dma_descriptor_ring *ring =
 	    kmalloc(sizeof(struct mite_dma_descriptor_ring), GFP_KERNEL);
 
-	if (ring == NULL)
-		return ring;
+	if (!ring)
+		return NULL;
 	ring->hw_dev = get_device(&mite->pcidev->dev);
-	if (ring->hw_dev == NULL) {
+	if (!ring->hw_dev) {
 		kfree(ring);
 		return NULL;
 	}