tsc2301 - switch to using input_dev->dev.parent

From similar drivers (Dmitry Torokhov <dtor@mail.ru>):

In preparation for struct class_device -> struct device input
core conversion, switch to using input_dev->dev.parent when
specifying device position in sysfs tree.

Signed-off-by: Klaus Pedersen <klaus.k.pedersen@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>