usb: dwc3: gadget: move % operation to increment helpers

By moving our % DWC3_NUM_TRB operation to the
increment helpers, the rest of the driver can be
simplified.

It's also a good practice to make sure we will have
a single place dealing with details about how to
increment our enqueue and dequeue pointers.

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