staging: exfat: Clean up return codes - FFS_FILEEXIST

Convert FFS_FILEEXIST to -EEXIST

Signed-off-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Link: https://lore.kernel.org/r/20191024155327.1095907-7-Valdis.Kletnieks@vt.edu


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