s390/qeth: don't clear the configured isolation mode

When qeth_set_access_ctrl_online() is called during the device's
initialization and discovers that isolation mode isn't supported, don't
clear the user's currently configured mode.
They intentionally choose to operate the device in this specific mode,
and degrading the isolation is not an option.

Only adjust the configuration when called via sysfs (ie. fallback = 1),
and here follow the common pattern and restore it from prev_isolation.

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