workqueue: Fix workqueue_execute_end() comment

workqueue_execute_end() is called after the callback function,
not before.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>