qeth: display "undefined" value of sysfs-attribute "layer2"

If nothing has been written into the qeth sysfs-attribute layer2,
its value is "-1" meaning "not yet defined". But the value is
displayed as "1" meaning "layer2 selected". The patch changes the
reading of this "-1"-value to "-1" to make clear the layer2-attribute
has not yet been defined.

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