The WWAN core not only multiplex the netdev configuration data, but
process it too, and needs some space to store its private data
associated with the netdev. Add a structure to keep common WWAN core
data. The structure will be stored inside the netdev private data before
WWAN driver private data and have a field to make it easier to access
the driver data. Also add a helper function that simplifies drivers
access to their data.

At the moment we use the common WWAN private data to store the WWAN data
link (channel) id at the time the link is created, and report it back to
user using the .fill_info() RTNL callback. This should help the user to
be aware which network interface is bound to which WWAN device data
channel.
Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
CC: M Chetan Kumar <m.chetan.kumar@intel.com>
CC: Intel Corporation <linuxwwan@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>