usb: dwc3: gadget: extract __dwc3_prepare_one_trb()

This new internal function will be used to solve a minor issue with dwc3
which exists in regards to short packets with OUT endpoints. Currently
we're asking gadget driver to *always* send us aligned requests; however
if we have enough TRBs we can easily append one extra TRB chained to the
previous and keep a throw away 1024 byte buffer around for that.

The actual fix will come in a separate patch, this is merely in
preparation for such fix.

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