usb: gadget: udc-xilinx: clean up a variable name

"ep->udc->lock" and "udc->lock" are the same thing.  It confuses Smatch
if we don't use the same name consistently.

Reviewed-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>