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