s390/qeth: split out OSN netdev ops

Rather than special-casing OSN in a number of places, just give this
device type its own netdev_ops structure.

When setting up the OSN net_device, also skip the handling of the
various HW offloads (eg TSO). The device shouldn't be advertising any of
them, and the OSN code paths in qeth don't have support for them.
In particular RX VLAN filtering is not supported, so don't hook up those
callbacks in the netdev_ops.

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