USB: dwc3: gadget: Add support for sending zero length packets

Currently driver is not honouring req->zero flag even though
function driver sets this flag. If device has to send IN data back
to host and length is multiple of endpoint maximum packet size, it
should send additional zero length packet at the end to terminate
transfer.

CRs-Fixed: 592504
Change-Id: I5a8aecc504098ed386a9a9d19f4f5b78c911a376
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>