qeth: crash during reboot after failing online setting

Online setting of a qeth device may fail for instance because of:
- out-of-memory condition when allocating qdio queues
- IDX ACTIVATE problem
- ...
Such a device is still returned in a driver_for_each_device loop
processed in qeth_reboot_event(), which calls
qeth_clear_qdio_buffers(). Make sure qeth_clear_output_buffer() is
called only, if the qdio queues have been successfully allocated
during initialization of a qeth device.

Signed-off-by: Ursula Braun <braunu@de.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>