staging: exfat: Clean up the namespace pollution part 3

These functions are only used in the local file, make them static

Signed-off-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Link: https://lore.kernel.org/r/20191112211238.156490-8-Valdis.Kletnieks@vt.edu


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>