staging: exfat: Correct return code

Use -ENOTEMPTY rather than -EEXIST for attempting to remove
a directory that still has files in it.

Signed-off-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Link: https://lore.kernel.org/r/20191112021000.42091-10-Valdis.Kletnieks@vt.edu


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