UPSTREAM: 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.

(cherry-picked from commit
d9261898a4b2c143c28568dc686a1becfc637a99)

Fixes: 04c03d10e507 ("usb: dwc3: gadget: handle request->zero")
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
Bug: 63867169
Change-Id: I97a1801c57dc169b6e9371d5aec599a14f316aff