memstick: convert struct class_device to struct device

struct class_device is going away, struct device should be used instead.

Signed-off-by: Tony Jones <tonyj@suse.de>
Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Cc: Alex Dubov <oakad@yahoo.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>