input/tps6507x-ts: use system_wq instead of dedicated workqueue

With cmwq, there's no reason to use a separate workqueue.  Drop
tps6507x_ts->wq and use system_wq instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Todd Fischer<todd.fischer@ridgerun.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: linux-input@vger.kernel.org
Cc: Dan Carpenter <error27@gmail.com>