mtd: nand: cleanup the nand_do_write_ops

nand_do_write_ops was broken in regard to writing several pages, each 
with its own oob.

Although nand_do_write_ops intends to allow such mode, it fails do do so
Probably this was never tested.

Also add missing checks for attempts to write at illegal offsets.

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>