s390/qeth: don't offer .ndo_bridge_* ops for OSA devices

qeth_l2_detect_dev2br_support() will only set brport_hw_features for IQD
devices. So qeth_l2_bridge_getlink() and qeth_l2_bridge_setlink() will
always return -EOPNOTSUPP on OSA devices. Just don't offer these
callbacks instead.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>