mtd: nand: tango: Use nand_to_mtd() instead of directly accessing chip->mtd

The nand_to_mtd() helper is here to hide internal mtd_info <-> nand_chip
association and ease future refactors.

Make use of this helper instead of directly accessing chip->mtd.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>