libertas: stop attempting to reset devices on unload

It wasn't working anyway -- by the time we get into if_usb_disconnect()
the USB core has already stopped us talking to the thing; even if it's
just on unload and the device still exists.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>