cmd: dfu: Propagate error if dfu gadget fails

On systems without usb gadget dfu core fails which was reported by error
but command itself returns pass which breaks any usage in a script.
That's why propagate error from run_usb_dnl_gadget().

Fixes: 16297cfb ("usb: new board-specific USB init interface")
Signed-off-by: Michal Simek <michal.simek@xilinx.com>