mtd: plat-nand: Fixup kerneldoc for struct platform_nand_chip

The set_parts and priv members of struct platform_nand_chip where
removed in commit c36a6ef3845262ade529afb9f458738b1f196f83 but the
kerneldoc wasn't updated.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Artem Bityutskiy <dedekind1@gmail.com>