usb: lthor: Specify correct parameter for sizeof type

This patch removes this warning:
  CC      drivers/usb/gadget/f_thor.o
drivers/usb/gadget/f_thor.c: In function ‘thor_tx_data’:
drivers/usb/gadget/f_thor.c:572:2: warning: format ‘%d’ expects argument
of type ‘int’, but argument 4 has type ‘long unsigned int’ [-Wformat=]
  debug("%s: dev->in_req->length:%d to_cpy:%d\n", __func__,
  ^

Signed-off-by: Michal Simek <michal.simek@xilinx.com>