usb: dwc3: gadget: Account for max size in TRB space

The current calculation takes dep->trb_dequeue - dep->trb_enqueue to
find the TRB space left.

If you enqueue 1, that results in:
(u8) 0 - (u8) 1 = 0xff = 255 TRBs left.

This is correct if DWC3_TRB_NUM == 256.

If DWC3_TRB_NUM is less than 256 (but still a power of 2) you need to
mod the result by DWC3_TRB_NUM.

For example the same calculation with DWC3_TRB_NUM = 8, results in:
255 % 6 = 7 TRBs left.
Signed-off-by: NJohn Youn <johnyoun@synopsys.com>
Signed-off-by: NFelipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc3/gadget.c

@@ -845,6 +845,7 @@ static void dwc3_prepare_one_trb(struct dwc3_ep *dep,
 static u32 dwc3_calc_trbs_left(struct dwc3_ep *dep)
 {
 	struct dwc3_trb		*tmp;
+	u8			trbs_left;
 
 	/*
 	 * If enqueue & dequeue are equal than it is either full or empty.
@@ -864,7 +865,10 @@ static u32 dwc3_calc_trbs_left(struct dwc3_ep *dep)
 		return DWC3_TRB_NUM - 1;
 	}
 
-	return dep->trb_dequeue - dep->trb_enqueue;
+	trbs_left = dep->trb_dequeue - dep->trb_enqueue;
+	trbs_left %= DWC3_TRB_NUM;
+
+	return trbs_left;
 }
 
 static void dwc3_prepare_one_trb_sg(struct dwc3_ep *dep,