usb: dwc3: gadget: Fix handling ZLP

The usb_request->zero doesn't apply for isoc. Also, if we prepare a
0-length (ZLP) TRB for the OUT direction, we need to prepare an extra
TRB to pad up to the MPS alignment. Use the same bounce buffer for the
ZLP TRB and the extra pad TRB.

Cc: <stable@vger.kernel.org> # v4.5+
Fixes: d6e5a549 ("usb: dwc3: simplify ZLP handling")
Fixes: 04c03d10 ("usb: dwc3: gadget: handle request->zero")
Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>