Input: mpr121-touchkey - drop redundant input_dev->dev.parent assignment

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

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