qeth: no recovery after layer mismatch (z/VM NICs)

Depending on their definition in z/VM, virtual devices for z/VM
VSWITCH or GuestLAN must be configured either in layer2 or in
layer3 mode. If qeth detects a layer mismatch, device activation
fails. Trying to recover from this error cannot help; thus
scheduling a recovery should be avoided.
In addition, since recovery is forbidden during online setting of
a qeth device, existence of its network device is guaranteed for all
dev_close() calls in qeth. The corresponding checks can be removed.

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>