mtd: Convert from class_device to device for MTD/mtdchar

struct class_device is going away, this converts the code to use struct
device instead.

Signed-off-by: Tony Jones <tonyj@suse.de>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>