1) Do not reset libertas devices upon module unload.  We're unloading
the module, we're not killing off devices.
2) Instead, reset libertas devices inside if_usb_disconnect, as we're
killing off interfaces and so on.
3) Resetting via disconnect callback means we no longer need to keep
track of probed libertas devices; kill off that list (and its wonderful
lack of locking..), as well.

Drop a useless comment as well.
Signed-off-by: Andres Salomon <dilinger@debian.org>
Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>