mtdraw: fix oob read and write

actually ops.ooboffs is not defaulted so when its value gets
added to chip->oob_poi in nand_fill_oob or nand_transfer_oob
the respective memcpy is using a wrong address.

With this patch, both md -s /dev/nandraw0 and cp xyz /dev/nandraw0.sb
are working fine on an i.MX28 target (instead of crashing the board).

Signed-off-by: Eric Bénard <eric@eukrea.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>