USB: gadget: f_fs: Use %zu when printing size_t

Using %lu for size_t gives compilation error on 32bit system.
Replace this with %zu which should be used to print size_t on
both 32bit and 64bit machines.

Change-Id: Id8a42dec3e081187054cb2353ba3473bae9dd547
CRs-fixed: 1069970
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>