Fixes the following use after free oops:

ehea: Reboot: freeing all eHEA resources
Unable to handle kernel paging request for data at address 0x6b6b6b6b6b6b6c5b
Faulting instruction address: 0xd000000000354488
cpu 0x0: Vector: 300 (Data Access) at [c00000002ec6f310]
    pc: d000000000354488: .ehea_shutdown_single_port+0x50/0x78 [ehea]
    lr: d00000000035447c: .ehea_shutdown_single_port+0x44/0x78 [ehea]
    sp: c00000002ec6f590
   msr: 8000000000009032
   dar: 6b6b6b6b6b6b6c5b
 dsisr: 40000000
  current = 0xc0000000281412e0
  paca    = 0xc0000000006df300
    pid   = 10930, comm = reboot
enter ? for help
[c00000002ec6f590] d00000000035d64c .ehea_remove+0x44/0x124 [ehea] (unreliable)
[c00000002ec6f630] c000000000319f88 .of_platform_device_remove+0x40/0x58
[c00000002ec6f6a0] c000000000291018 .__device_release_driver+0xb0/0xf0
[c00000002ec6f730] c000000000291120 .driver_detach+0xc8/0xfc
[c00000002ec6f7c0] c00000000028fe24 .bus_remove_driver+0xb4/0x114
[c00000002ec6f850] c000000000291768 .driver_unregister+0x54/0x74
[c00000002ec6f8e0] c00000000031a0c8 .of_unregister_driver+0x14/0x28
[c00000002ec6f950] c000000000023ba0 .ibmebus_unregister_driver+0x10/0x24
[c00000002ec6f9c0] d000000000354180 .ehea_reboot_notifier+0x30/0x4c [ehea]
[c00000002ec6fa40] c0000000003c95a8 .notifier_call_chain+0x5c/0xcc
[c00000002ec6fae0] c000000000082cd4 .__blocking_notifier_call_chain+0x70/0xb0
[c00000002ec6fb90] c000000000075cf8 .kernel_restart_prepare+0x24/0x58
[c00000002ec6fc10] c000000000075f0c .kernel_restart+0x20/0x6c
[c00000002ec6fc90] c000000000078674 .sys_reboot+0x1d4/0x290
[c00000002ec6fe30] c0000000000086ac syscall_exit+0x0/0x40
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>