usb: gadget: net2280: use swap() instead of open coding it

Address the following coccicheck warning:
drivers/usb/gadget/udc/net2280.c:940:20-21: WARNING opportunity for swap().
drivers/usb/gadget/udc/net2280.c:944:25-26: WARNING opportunity for swap().

by using swap() for the swapping of variable values and drop the tmp
variables (`tmp` and `end`) that are not needed any more.
Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Link: https://lore.kernel.org/r/20220407100459.3605-1-guozhengkui@vivo.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/udc/net2280.c

@@ -932,19 +932,11 @@ static void start_dma(struct net2280_ep *ep, struct net2280_request *req)
 static inline void
 queue_dma(struct net2280_ep *ep, struct net2280_request *req, int valid)
 {
-	struct net2280_dma	*end;
-	dma_addr_t		tmp;
-
 	/* swap new dummy for old, link; fill and maybe activate */
-	end = ep->dummy;
-	ep->dummy = req->td;
-	req->td = end;
-
-	tmp = ep->td_dma;
-	ep->td_dma = req->td_dma;
-	req->td_dma = tmp;
+	swap(ep->dummy, req->td);
+	swap(ep->td_dma, req->td_dma);
 
-	end->dmadesc = cpu_to_le32 (ep->td_dma);
+	req->td->dmadesc = cpu_to_le32 (ep->td_dma);
 
 	fill_dma_desc(ep, req, valid);
 }