qeth: avoid crash after detach of replugged device

If a qeth device is plugged off, setting the device online stops in
state HARDSETUP and a failure is reported to the base cio-layer
causing halt/clear to be invoked. Replugging the device again triggers
a qeth recovery without notification of the cio-layer. If a device
is ungrouped in this state, the qeth set_offline function is not
invoked, because the corresponding ccwgroup device is not in state
ONLINE. Then incoming traffic is still handled by the qdio layer
resulting in a crash in qeth_l<x>_qdio_input_handler, because (part
of) the qeth data structures for this device are already removed.
Solution: After replugging the device qeth recovery should lead to a
working net device. Thus a "LAN offline" result when setting a qeth
device online must not report a failure to the base cio-layer.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>