dwc2: gadget: Fix completed transfer size calculation in DDMA

Fix calculation of transfer size on completion in function
dwc2_gadget_get_xfersize_ddma().

Added increment of descriptor pointer to move to next descriptor in
the loop.

Fixes: aa3e8bc8 ("usb: dwc2: gadget: DDMA transfer start and complete")

Signed-off-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>