bus: arm-ccn: fix module unloading Error: Removing state 147 which has instances left.

Unregistering the driver before calling cpuhp_remove_multi_state() removes
any remaining hotplug cpu instances so __cpuhp_remove_state_cpuslocked()
doesn't emit this warning:

[  268.748362] Error: Removing state 147 which has instances left.
[  268.748373] ------------[ cut here ]------------
[  268.748386] WARNING: CPU: 2 PID: 5476 at kernel/cpu.c:1734 __cpuhp_remove_state_cpuslocked+0x454/0x4f0
[  268.748389] Modules linked in: arm_ccn(-) [last unloaded: arm_ccn]
[  268.748403] CPU: 2 PID: 5476 Comm: rmmod Tainted: G        W       4.14.0-rc4+ #3
[  268.748406] Hardware name: AMD Seattle/Seattle, BIOS 10:18:39 Dec  8 2016
[  268.748410] task: ffff8001a18ca000 task.stack: ffff80019c120000
[  268.748416] PC is at __cpuhp_remove_state_cpuslocked+0x454/0x4f0
[  268.748421] LR is at __cpuhp_remove_state_cpuslocked+0x448/0x4f0
[  268.748425] pc : [<ffff2000081729ec>] lr : [<ffff2000081729e0>] pstate: 60000145
[  268.748427...