usb: gadget: udc-xilinx: remove redundant pointer udc

Pointer udc is assigned but never read, hence it is redundant and
can be removed. Cleans up clang warning:

drivers/usb/gadget/udc/udc-xilinx.c:974:2: warning: Value stored
to 'udc' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>