b43: remove Comparison of 0/1 to bool variable in pio.c

Fix the following coccicheck warning:

drivers/net/wireless/broadcom/b43/pio.c:768:10-25: WARNING: Comparison
of 0/1 to bool variable
Signed-off-by: NJason Yan <yanaijie@huawei.com>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200504113311.41026-1-yanaijie@huawei.com

drivers/net/wireless/broadcom/b43/pio.c

@@ -765,7 +765,7 @@ void b43_pio_rx(struct b43_pio_rxqueue *q)
 	bool stop;
 
 	while (1) {
-		stop = (pio_rx_frame(q) == 0);
+		stop = !pio_rx_frame(q);
 		if (stop)
 			break;
 		cond_resched();