mtd: use class_groups instead of class_attrs

The class_attrs pointer is long depreciated, and is about to be finally
removed, so move to use the class_groups pointer instead.

Cc: Artem Bityutskiy <dedekind1@gmail.com>
Acked-by: Richard Weinberger <richard@nod.at>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>
Cc: <linux-mtd@lists.infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>