mtd: cleanup mtd_oobtest

- Remove unnecessary memset for bbt
  All entries will be initialized at a few lines below
- Remove unnecessary initialization for mtd->erasesize
- Use write_whole_device()

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>