usb: gadget: udc: Potential Oops in error handling code

If this is in "transceiver" mode the the ->qwork isn't required and is
a NULL pointer.  This can lead to a NULL dereference when we call
destroy_workqueue(udc->qwork).

Fixes: 3517c31a ("usb: gadget: mv_udc: use devm_xxx for probe")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>