Staging: rt2870: Revert d44ca7 Removal of kernel_thread() API

Staging: rt2870: Revert d44ca7 Removal of kernel_thread() API

The sanity check this patch introduced triggers on shutdown, apparently due to
threads having already exited by the time BUG_ON() is reached.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Cc: Peter Teoh <htmldeveloper@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>