Input: mma8450 - add parent device

Add the parent device so that udev can show the full hierarchy. This avoids
the device showing up under /devices/virtual/input instead of the i2c bus
it is actually attached to.

Signed-off-by: Stefan Sauer <ensonic@google.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>