usb: dwc3: gadget: simplify __dwc3_gadget_kick_transfer()

as it turns out, we don't need the extra 'start_new'
argument as that can be inferred from DWC3_EP_BUSY
flag.

Because of that, we can simplify
__dwc3_gadget_kick_transfer() by quite a bit, even
allowing us to prepare more TRBs unconditionally.

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