usb: dwc3: ep0: fix for dead code

commit 6856d30c (usb: dwc3: ep0: return early
on NULL requests) tried to fix a minor corner
case where we could dereference a NULL pointer
but it also ended up introducing some dead code.

Unfortunately, that dead code, if reached, could
end up starving the endpoint request list because
a request would never be given back when it should.

Fix this by moving the check for empty request list
before its first use.
Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/dwc3/ep0.c

@@ -791,6 +791,10 @@ static void dwc3_ep0_complete_data(struct dwc3 *dwc,
 
 	trb = dwc->ep0_trb;
 
+	r = next_request(&ep0->request_list);
+	if (!r)
+		return;
+
 	status = DWC3_TRB_SIZE_TRBSTS(trb->size);
 	if (status == DWC3_TRBSTS_SETUP_PENDING) {
 		dwc3_trace(trace_dwc3_ep0, "Setup Pending received");
@@ -801,10 +805,6 @@ static void dwc3_ep0_complete_data(struct dwc3 *dwc,
 		return;
 	}
 
-	r = next_request(&ep0->request_list);
-	if (!r)
-		return;
-
 	ur = &r->request;
 
 	length = trb->size & DWC3_TRB_SIZE_MASK;