libertas: only enable mesh when interface is active

Previously, the mesh was running whenever the appropriate hardware
and firmware was present.

Now we only run the mesh when the interface is running.

Also simplifies interface management a little.

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