libertas: reset devices upon disconnect rather than module unloading

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>