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

Use '%z' format specifier for sizeof operator instead
of '%u' to fix build warnings like:

    warning: format '%u' expects type 'unsigned int', but argument 3 has type 'long unsigned int'

Change-Id: If016ee2e47a97773ae4f27d88d79a886b0acf0bc
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>