s390/qeth: fix false reporting of VNIC CHAR config failure

Symptom: Error message "Configuring the VNIC characteristics failed"
in dmesg whenever an OSA interface on z15 is set online.

The VNIC characteristics get re-programmed when setting a L2 device
online. This follows the selected 'wanted' characteristics - with the
exception that the INVISIBLE characteristic unconditionally gets
switched off.

For devices that don't support INVISIBLE (ie. OSA), the resulting
IO failure raises a noisy error message
("Configuring the VNIC characteristics failed").
For IQD, INVISIBLE is off by default anyways.

So don't unnecessarily special-case the INVISIBLE characteristic, and
thereby suppress the misleading error message on OSA devices.

Fixes: caa1f0b1

 ("s390/qeth: add VNICC enable/disable support")
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>