usb: dwc2: gadget: fix zero length packet transfers

According to programming guide, zero length packet should be
programmed on its own and should not be counted in DIEPTSIZ.PktCnt
with other packets.

For ep0, this is the zlp for DATA IN stage (if required) and not for
the STATUS stage.

Tested-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Gregory Herrero <gregory.herrero@intel.com>
Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>