workqueue.h: remove loops of single statement macros

checkpatch.pl complained about two single statement macros in
do while (0) loops.  The loops and the trailing semicolons are
now removed, which makes checkpatch happy and the two macros
consistent with the rest of the file.

Signed-off-by: Valentin Rothberg <valentinrothberg@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>