usb: dwc3: gadget: simplify dwc3_ep_prev_trb()

We always need to decrement our index by at least
one. Simplify the implementation by using a
temporary local variable and making sure that we
will always decrement one extra if tmp == 0.
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc3/gadget.c

@@ -845,12 +845,12 @@ static void dwc3_prepare_one_trb(struct dwc3_ep *dep,
  */
 static struct dwc3_trb *dwc3_ep_prev_trb(struct dwc3_ep *dep, u8 index)
 {
-	if (!index)
-		index = DWC3_TRB_NUM - 2;
-	else
-		index = dep->trb_enqueue - 1;
+	u8 tmp = index;
+
+	if (!tmp)
+		tmp = DWC3_TRB_NUM - 1;
 
-	return &dep->trb_pool[index];
+	return &dep->trb_pool[tmp - 1];
 }
 
 static u32 dwc3_calc_trbs_left(struct dwc3_ep *dep)