mtd: nand: Get rid of the mtd parameter in all auto-detection functions

Now that struct nand_chip embeds an mtd_info object we can get rid of the
mtd parameter and extract it from the chip parameter with the nand_to_mtd()
helper.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>