usb: dwc3: gadget: Refactor preparing extra TRB

When the driver prepares the extra TRB, it uses bounce buffer. If we
just add a new parameter to dwc3_prepare_one_trb() to indicate this,
then we can refactor and simplify the driver quite a bit.
dwc3_prepare_one_trb() also checks if a request had been moved to the
started list. This is a prerequisite to subsequence patches improving
the handling of extra TRBs.

Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>