HID: hid-mosart: ignore buttons report

This commit allows the device to be recognized as a touchscreen, and not a
touchpad by xf86-input-evdev.

The device has 2 modes. The first one is an emulation of a touchscreen by
sending left and right button, and the second mode is the one used in
dual-touch (sending trackingID, touch and else).

That's why there is a hid report containing left and right buttons
(9000001 and 9000002). The point is that xorg relies on these fields to
determine if it's a touchpad or a touchscreen.
Clearing the report (return -1) makes xorg detecting it out of the box
as a quite pleasant (dual)touchscreen.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@enac.fr>
Acked-by: Chase Douglas <chase.douglas@canonical.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>