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