[MTD] NAND: OOB buffer offset fixups

In the case of data-pad-ecc-pad-data... layout the oob start position
has to be sizeof(data) in nand_write_oob_syndrom().

In nand_fill_oob() we need to copy to buf + buffer offset instead of
buf + write offset.

From: Vitaly Wool <vwool@ru.mvista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>