staging: exfat: cleanup explicit comparisons to NULL

Fixes checkpatch.pl warnings:

  CHECK: Comparison to NULL could be written "expr"
  CHECK: Comparison to NULL could be written "!expr"

Signed-off-by: Valentin Vidic <vvidic@valentin-vidic.from.hr>
Link: https://lore.kernel.org/r/20190903205659.18856-1-vvidic@valentin-vidic.from.hr


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