staging: exfat: use BIT macro for defining sizes

Fixes checkpatch.pl warning:

  CHECK: Prefer using the BIT macro

Signed-off-by: Valentin Vidic <vvidic@valentin-vidic.from.hr>
Link: https://lore.kernel.org/r/20190902184319.11971-1-vvidic@valentin-vidic.from.hr


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