usb: gadget: dwc2: fix zlp handling

The patch 10209abe
usb: dwc2: gadget: Add scatter-gather mode

avoided a NULL pointer dereference (hs_ep->req == NULL) by
calling dwc2_gadget_fill_nonisoc_xfer_dma_one() directly instead of through
the dwc2_gadget_config_nonisoc_xfer_ddma() wrapper, which unconditionally
dereferenced the said pointer.

However, this was based on an incorrect assumption that in the context of
dwc2_hsotg_program_zlp() the pointer is always NULL, which is not the case.
The result were SB CV MSC tests failing starting from Test Case 6.

Instead, this patch reverts to calling the wrapper and adds a check for
the pointer being NULL inside the wrapper.

Fixes: 10209abe

 (usb: dwc2: gadget: Add scatter-gather mode)
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>