usb: musb: host: 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: NAjay Kumar Gupta <ajay.gupta@ti.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

drivers/usb/musb/musb_host.c

@@ -693,6 +693,8 @@ static void musb_ep_program(struct musb *musb, u8 epnum,
 	void __iomem		*epio = hw_ep->regs;
 	struct musb_qh		*qh = musb_ep_get_qh(hw_ep, !is_out);
 	u16			packet_sz = qh->maxpacket;
+	u8			use_dma = 1;
+	u16			csr;
 
 	dev_dbg(musb->controller, "%s hw%d urb %p spd%d dev%d ep%d%s "
 				"h_addr%02x h_port%02x bytes %d\n",
@@ -704,9 +706,17 @@ static void musb_ep_program(struct musb *musb, u8 epnum,
 
 	musb_ep_select(mbase, epnum);
 
+	if (is_out && !len) {
+		use_dma = 0;
+		csr = musb_readw(epio, MUSB_TXCSR);
+		csr &= ~MUSB_TXCSR_DMAENAB;
+		musb_writew(epio, MUSB_TXCSR, csr);
+		hw_ep->tx_channel = NULL;
+	}
+
 	/* candidate for DMA? */
 	dma_controller = musb->dma_controller;
-	if (is_dma_capable() && epnum && dma_controller) {
+	if (use_dma && is_dma_capable() && epnum && dma_controller) {
 		dma_channel = is_out ? hw_ep->tx_channel : hw_ep->rx_channel;
 		if (!dma_channel) {
 			dma_channel = dma_controller->channel_alloc(