staging: brcm80211: remove kernel_thread() for wl_iscan_thread and wl_event_thread.

Replace kernel_thread() with kthread_run().  Replace pid with tsk,
and exited with kthread_stop()/kthread_should_stop().

event_tsk, and tsk are NULL when their respective threads are not running.

Signed-off-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>