wakelock: use single threaded workqueueus

Avoid the concurrency management that could delay execution if there's
lots of workqueue traffic (and allow the runqueue to run on any CPU).

Lots of workqueue traffic can be generated by setting
ondemand/sampling_rate to 10000.

Change-Id: I55b8a223f4a389b93d49eef0d809eb1efe1790ba

Signed-off-by: Andrea Arcangeli <andrea@cpushare.com>