HID: multitouch: Combine all left-button events in a frame

According to the Win8 Precision Touchpad spec, inside the HID_UP_BUTTON
usage-page usage 1 is for a clickpad getting clicked, 2 for an external
left button and 3 for an external right button. Since Linux uses
BTN_LEFT for a clickpad being clicked we end up mapping both usage 1
and 2 to BTN_LEFT and if a single report contains both then we ended
up always reporting the value of both in a single SYN, e.g. :
BTN_LEFT 1, BTN_LEFT 0, SYN. This happens for example with Hantick
HTT5288 i2c mt touchpads.

This commit fixes this by not immediately reporting left button when we
parse the report, but instead storing or-ing together the values and
reporting the result from mt_sync_frame() when we've a complete frame.

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