staging: exfat: Update the TODO file

Updating with the current laundry list of things that need attention.

Signed-off-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Link: https://lore.kernel.org/r/20191112223609.163501-1-Valdis.Kletnieks@vt.edu


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