ntb_netdev: Simplify remove with client device drvdata

Replace the elaborate private structure global linked-list used in
ntb_netdev_probe() and ntb_netdev_remove() by stashing our private
data in the NTB transport client device.

Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>