libertas: don't exit worker thread until kthread_stop() is called

The kthread code can't cope with a thread exiting of its own accord and
then someone calling kthread_stop() for it. When the thread detects that
it needs to die, make it wait for kthread_stop() to be called.

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