mtd: nand: make use of nand_set/get_controller_data() helpers

New helpers have been added to avoid directly accessing chip->field. Use
them where appropriate.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
[Brian: fixed a few rebase conflicts]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>