kinect_upload_fw: fix -Wmissing-prototypes warning

kinect_upload_fw.c:60:6: warning: no previous prototype for ‘dump_bl_cmd’ [-Wmissing-prototypes]
kinect_upload_fw.c:67:5: warning: no previous prototype for ‘get_first_reply’ [-Wmissing-prototypes]
kinect_upload_fw.c:86:5: warning: no previous prototype for ‘get_reply’ [-Wmissing-prototypes]

GCC emits "missing-prototypes" argument for _public_ functions without a
prototype, making them static is the right fix here.