Input: edt-ft54x6: Clean up timer and workqueue on remove

If no interrupt is defined then a timer and workqueue are used
to poll the controller.
On remove these were not being cleaned up correctly.

Fixes: ca61fdab "Input: edt-ft5x06: Poll the device if no interrupt is
configured."

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>