HID: multitouch: fix touchpad buttons

Commit "HID: multitouch: use the callback "report" instead..." breaks the
buttons of touchpads following the HID multitouch specification.
The buttons were emmitted through hid-input, but as now the events
are generated only in hid-multitouch, the buttons are not emmitted anymore.

The input_event() call is far much simpler than the hid-input one as
many of the different tests do not apply to multitouch touchpads.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>