qeth: use -EOPNOTSUPP instead of -ENOTSUPP.

return value -ENOTSUPP is not valid in userspace context, use
-EOPNOTSUPP instead.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>