staging: exfat: Clean up return code - FFS_MEMORYERR

Convert FFS_MEMORYERR to -ENOMEM

Signed-off-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Link: https://lore.kernel.org/r/20191024155327.1095907-9-Valdis.Kletnieks@vt.edu


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