usb: dwc2: avoid NULL dereferences

For unidirectional endpoints, the endpoint pointer will be NULL for the
unused direction.  Check that the endpoint is active before
dereferencing this pointer.

Fixes: 1b4977c7 ("usb: dwc2: Update dwc2_handle_incomplete_isoc_in() function")
Fixes: 689efb26 ("usb: dwc2: Update dwc2_handle_incomplete_isoc_out() function")
Fixes: d8484552 ("usb: dwc2: Update GINTSTS_GOUTNAKEFF interrupt handling")
Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>