s390/qeth: only init the isolation mode when necessary

A newly initialized device defaults to ISOLATION_MODE_NONE, don't bother
with programming this a second time.

Then remove the OSD/OSX check, it's already done in the sysfs path
whenever the user actually changes the configuration.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>