Revert "usb: dwc3: gadget: always decrement by 1"

This reverts commit 6f8245b4 ("usb: dwc3: gadget: always decrement
by 1").

We can't always decrement this value.

We should decrement only if the calculation of free slots results in a
LINK TRB being among one of the free slots (dequeue < enqueue).

Otherwise, if the LINK TRB is not among the free slots then it should
not be decremented.
Signed-off-by: NJohn Youn <johnyoun@synopsys.com>
Signed-off-by: NFelipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc3/gadget.c

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