usb: musb: gadget: don't program dma for zero byte tx

This is to reduce the overhead of dma programming for zero byte
transmit as same can be done using pio mode.

Signed-off-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/musb/musb_gadget.c

@@ -373,7 +373,7 @@ static void txstate(struct musb *musb, struct musb_request *req)
 		request_size = min_t(size_t, request->length - request->actual,
 					musb_ep->dma->max_len);
 
-		use_dma = (request->dma != DMA_ADDR_INVALID);
+		use_dma = (request->dma != DMA_ADDR_INVALID && request_size);
 
 		/* MUSB_TXCSR_P_ISO is still set correctly */