ivtv: use kthread_worker instead of workqueue

Upcoming workqueue updates will no longer guarantee fixed workqueue to
worker kthread association, so giving RT priority to the irq worker
won't work.  Use kthread_worker which guarantees specific kthread
association instead.  This also makes setting the priority cleaner.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Andy Walls <awalls@md.metrocast.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: ivtv-devel@ivtvdriver.org
Cc: linux-media@vger.kernel.org