This is the first step of a long series moving members between
isdn_net_local and isdn_net_dev.

Today, a one-to-one relationship between these both structures exist, so
it does not really matter where the members live. However, the goal
is to get a correspondence like

net_device -> isdn_net_local -> master isdn_net_device
                                           |
                                slave  isdn_net_device
                                           |
                                slave  isdn_net_device

where more than one isdn_net_device can exist per actual net_device,
due to channel bundling.