trivial: NULL noise: drivers/mtd/tests/mtd_*test.c

Fix this sparse warnings:
  drivers/mtd/tests/mtd_oobtest.c:139:33: warning: Using plain integer as NULL pointer
  drivers/mtd/tests/mtd_oobtest.c:192:33: warning: Using plain integer as NULL pointer
  drivers/mtd/tests/mtd_oobtest.c:219:41: warning: Using plain integer as NULL pointer
  drivers/mtd/tests/mtd_oobtest.c:284:25: warning: Using plain integer as NULL pointer
  drivers/mtd/tests/mtd_oobtest.c:525:25: warning: Using plain integer as NULL pointer
  drivers/mtd/tests/mtd_oobtest.c:545:25: warning: Using plain integer as NULL pointer
  drivers/mtd/tests/mtd_oobtest.c:569:33: warning: Using plain integer as NULL pointer
  drivers/mtd/tests/mtd_oobtest.c:589:33: warning: Using plain integer as NULL pointer
  drivers/mtd/tests/mtd_oobtest.c:613:33: warning: Using plain integer as NULL pointer
  drivers/mtd/tests/mtd_oobtest.c:633:33: warning: Using plain integer as NULL pointer
  drivers/mtd/tests/mtd_oobtest.c:673:41: warning: Using plain integer as NULL pointer
  drivers/mtd/tests/mtd_oobtest.c:701:33: warning: Using plain integer as NULL pointer
  drivers/mtd/tests/mtd_readtest.c:74:41: warning: Using plain integer as NULL pointer

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Acked-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>