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: I8b3b57a1e2664367c685e9f3d6758f7ea39c4b56
Git-Commit: 928b59a553baea2317a288dea5d957f9a9a9e8b6
Git-repo: https://source.codeaurora.org/quic/la/kernel/msm-4.9


Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Sivasri Kumar Vanka <sivasri@codeaurora.org>