Input: twl6040-vibra - fix NULL pointer dereference by removing workqueue

commit 21fb9f0d ("Input: twl6040-vibra - use system workqueue")

says that it switches to use the system workqueue but it did neither

- remove the workqueue struct variable
- replace code to really use the system workqueue

Instead it calls queue_work() on uninitialized info->workqueue.

The result is a NULL pointer dereference in vibra_play().

Solution: use schedule_work

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>