Input: synaptics - use SERIO_OOB_DATA to handle trackstick buttons

Instead of using custom method of "passing" extended buttons from the
touchpad to trackstick, let's switch to the newly introduced SERIO_OOB_DATA
channel.

Tested-By: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>