usb: dwc3: gadget: Fix truncated cast issue

From sparse:
warning: cast truncates bits from constant value (100 becomes 0)

The DWC3_TRB_NUM constant is too big for u8. Do the calculation a
slightly different way that should still be optimized out for the case
where DWC3_TRB_NUM == 256.
Signed-off-by: NJohn Youn <johnyoun@synopsys.com>
Signed-off-by: NFelipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc3/gadget.c

@@ -882,7 +882,7 @@ static u32 dwc3_calc_trbs_left(struct dwc3_ep *dep)
 	}
 
 	trbs_left = dep->trb_dequeue - dep->trb_enqueue;
-	trbs_left %= DWC3_TRB_NUM;
+	trbs_left &= (DWC3_TRB_NUM - 1);
 
 	if (dep->trb_dequeue < dep->trb_enqueue)
 		trbs_left--;