Input: wm831x-on - drop redundant wm831x_on->dev->dev.parent assignment

devm_input_allocate_device() already initializes
wm831x_on->dev->dev.parent to the supplied parent device, so the
explicit assignment in wm831x_on_probe() is redundant. Remove it.

Assisted-by: LLM
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>