usb: dwc3: gadget: don't send extra ZLP

If the request->length is zero, a ZLP should already be sent due to that
and another ZLP is not needed to terminate the transfer.

Fixes: 04c03d10e507 ("usb: dwc3: gadget: handle request->zero")
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Change-Id: I747a9fd710d90986c7befd3d5ee5a1a5fceeaaed
Git-commit: d9261898a4b2c143c28568dc686a1becfc637a99
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>