usb: gadget: u_ether: use %z format specifier for size_t

Use '%zd' format specifier for size_t type instead
of '%d' to fix build warnings like:

drivers/usb/gadget/function/u_ether.c: In function ‘rx_submit’:
drivers/usb/gadget/function/u_ether.c:244:2: warning: format ‘%d’ expects argument of type ‘int’, but argument 4 has type ‘size_t’ [-Wformat=]
  DBG(dev, "%s: size: %d\n", __func__, size);
  ^

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>