mtd: nand: fully initialize mtd_oob_ops

We're not initializing the ooblen field. Our users don't care, since
they check that oobbuf == NULL first, but it's good practice to zero
unused fields out.

We can drop the NULL initializations since we're memset()ing the whole
thing.

Noticed by Coverity, CID #200821, #200822

Signed-off-by: Brian Norris <computersforpeace@gmail.com>