usb: dwc2: gadget: fix packet drop issue in dwc2_gadget_handle_nak

In ISOC transfer, when the NAK interrupt happens, we shouldn't complete
a usb request, the current flow will complete one usb request with no
hardware transfer, this will lead to a packet drop on the usb bus.

Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Zeng Tao <prime.zeng@hisilicon.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>