usb: cdns3: gadget: calculate TD_SIZE based on TD

The TRB entry TD_SIZE is the packet number for the TD (request) but not the
each TRB, so it only needs to be assigned for the first TRB during the TD,
and the value of it is for TD too.

Fixes: 7733f6c3 ("usb: cdns3: Add Cadence USB3 DRD Driver")
Signed-off-by: Peter Chen <peter.chen@nxp.com>