libertas: link mesh device to wiphy

The mesh device is now exposed as an interface of the wiphy.
This exposes the mesh device to the cfg80211 interface, allowing
mesh channel selection to be reimplemented, and available to
NetworkManager as it was before.

Some header tweaking was needed in order to implement lbs_mesh_activated().

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>