usb: dwc3: gadget: Handle 0 xfer length for OUT EP

For OUT endpoints, zero-length transfers require MaxPacketSize buffer as
per the DWC_usb3 programming guide 3.30a section 4.2.3.3.

This patch fixes this by explicitly checking zero length
transfer to correctly pad up to MaxPacketSize.

Fixes: c6267a51 ("usb: dwc3: gadget: align transfers to wMaxPacketSize")
Cc: stable@vger.kernel.org
Signed-off-by: Tejas Joglekar <joglekar@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc3/gadget.c

@@ -1119,7 +1119,7 @@ static void dwc3_prepare_one_trb_linear(struct dwc3_ep *dep,
 	unsigned int maxp = usb_endpoint_maxp(dep->endpoint.desc);
 	unsigned int rem = length % maxp;
 
-	if (rem && usb_endpoint_dir_out(dep->endpoint.desc)) {
+	if ((!length || rem) && usb_endpoint_dir_out(dep->endpoint.desc)) {
 		struct dwc3	*dwc = dep->dwc;
 		struct dwc3_trb	*trb;