usb: dwc3: gadget: always decrement by 1

We need to decrement in both cases (enq > deq and
enq < deq)

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc3/gadget.c

@@ -884,12 +884,9 @@ static u32 dwc3_calc_trbs_left(struct dwc3_ep *dep)
 		return DWC3_TRB_NUM - 1;
 	}
 
-	trbs_left = dep->trb_dequeue - dep->trb_enqueue;
+	trbs_left = dep->trb_dequeue - dep->trb_enqueue - 1;
 	trbs_left &= (DWC3_TRB_NUM - 1);
 
-	if (dep->trb_dequeue < dep->trb_enqueue)
-		trbs_left--;
-
 	return trbs_left;
 }