ubi: ubi.h: fix kernel-doc warnings

Fix all default warning level kernel-doc warnings in ubi.h:

Warning: drivers/mtd/ubi/ubi.h:176 struct member 'u' not described in 'ubi_wl_entry'
Warning: drivers/mtd/ubi/ubi.h:370 struct member 'is_dead' not described in 'ubi_volume'
Warning: drivers/mtd/ubi/ubi.h:655 struct member 'is_dead' not described in 'ubi_device'
Warning: drivers/mtd/ubi/ubi.h:743 bad line: all existing Fastmap data structures
Warning: drivers/mtd/ubi/ubi.h:783 struct member 'vidb' not described in 'ubi_attach_info'
Warning: drivers/mtd/ubi/ubi.h:1098 expecting prototype for ubi_init_vid_buf().
 Prototype was for ubi_alloc_vid_buf() instead

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>