ntb_netdev: set the net_device's parent

At present, ntb_netdev devices end up under /sys/devices/virtual/net
completely unconnected to the ntb trees below them. This patch sets the
parent of the net_device (using SET_NETDEV_DEV) to the client_dev
device. This results in a better connected sysfs path for the network
device:

/sys/devices/pci0000:00/0000:00:03.0/0000:03:00.1/0000:03:00.1/ntb_netdev0/net/eth2

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Acked-by: Allen Hubbe <Allen.Hubbe@dell.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>