veth: always report zero combined channels

veth get_channel currently reports for channels being both RX/TX and
combined. As Jakub noted:

"""
ethtool man page is relatively clear, unfortunately the kernel code
is not and few read the man page. A channel is approximately an IRQ,
not a queue, and IRQ can't be dedicated and combined simultaneously
"""

This patch changes the information exposed by veth_get_channels,
setting max_combined to zero, being more consistent with the above
statement. The ethtool_channels is always cleared by the caller, we just
need to avoid setting the 'combined' fields.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>