s390/qeth: support early setup for z/VM NICs

The transport mode that a z/VM NIC is configured in, must match the
hypervisor-internal network which the NIC is coupled to.

To get this right automatically, have qeth issue a diag26c hypervisor call
that provides all sorts of information for a specific VNIC.
With z/VM update VM65918, this also includes the VNIC's required
transport mode.

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