This patch cleans up the manual device_create_file() or
class_create_file() calls by replacing with static attribute groups.
It simplifies the code and also avoids the possible races between the
device/class registration and sysfs creations.

For the simplification, also make ubi_class a static instance with
initializers, too.

Amend a bit by Hujianyang.
Signed-off-by: NTakashi Iwai <tiwai@suse.de>
Tested-by: NSheng Yong <shengyong1@huawei.com>
Signed-off-by: Nhujianyang <hujianyang@huawei.com>
Signed-off-by: NRichard Weinberger <richard@nod.at>