usb: dwc3: gadget: Simplify skipping of link TRBs

Make the skipping of the link TRBS built-in to the increment operation.
This simplifies the code wherever we increment the trb index and ensures
that we never end up pointing to a link trb.

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