s390/qeth: support SG for more device types

NETIF_F_SG support is currently limited to OSA (and for L2 even OSD)
devices. Advertise it for some more device types (OSM, L2 OSX, z/VM OSA)
that share the same code paths. For now, keep it switched off by
default on these devices.

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