s390/ap: fix ap devices reference counting

s390/ap: fix ap devices reference counting

With the last rework of the AP bus scan function one get_device() is
missing causing the reference counter to be one instance too
low. Together with binding/unbinding device drivers to an ap device it
may end up in an segfault because the ap device is freed but a device
driver still assumes it's pointer to the ap device is valid:

Unable to handle kernel pointer dereference in virtual kernel address space
Failing address: 6b6b6b6b6b6b6000 TEID: 6b6b6b6b6b6b6803
Fault in home space mode while using kernel ASCE.
Krnl PSW : 0404e00180000000 000000001472f3b6 (klist_next+0x7e/0x180)
           R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3
Call Trace:
 [<000000001472f3b6>] klist_next+0x7e/0x180
([<000000001472f36a>] klist_next+0x32/0x180)
 [<00000000147c14de>] bus_for_each_dev+0x66/0xb8
 [<0000000014aab0d4>] ap_scan_adapter+0xcc/0x6c0
 [<0000000014aab74a>] ap_scan_bus+0x82/0x140
 [<0000000013f3b654>] process_one_work+0x27c/0x478
 [<0000000013f3b8b6>] worker_thread+0x66/0x368
 [<0000000013f44e32>] kthread+0x17a/0x1a0
 [<0000000014af23e4>] ret_from_fork+0x24/0x2c
Kernel panic - not syncing: Fatal exception: panic_on_oops

Fixed by adjusting the reference count with get_device() on the right
place. Also now the device drivers don't need to adjust the ap
device's reference counting any more. This is now done in the ap bus
probe and remove functions.

Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Fixes: 4f2fcccdb547 ("s390/ap: add card/queue deconfig state")
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>