netiucv: cleanup attribute usage

Let the driver core handle device attribute creation and removal. This
will simplify the code and eliminates races between attribute
availability and userspace notification via uevents.

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