The 'ublk_chr_class' is needed when deleting ublk char devices in
ublk_exit(), so move it after devices(idle) are removed.

Fixes the following warning reported by Harris, James R:

[  859.178950] sysfs group 'power' not found for kobject 'ublkc0'
[  859.178962] WARNING: CPU: 3 PID: 1109 at fs/sysfs/group.c:278 sysfs_remove_group+0x9c/0xb0
Reported-by: "Harris, James R" <james.r.harris@intel.com>
Fixes: 71f28f31 ("ublk_drv: add io_uring based userspace block driver")
Link: https://lore.kernel.org/linux-block/Y9JlFmSgDl3+zy3N@T590/T/#tSigned-off-by: Ming Lei <ming.lei@redhat.com>
Tested-by: Jim Harris <james.r.harris@intel.com>
Link: https://lore.kernel.org/r/20230126115346.263344-1-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>