HID: lenovo: Hide middle-button press until release

Don't relay a middle button press to userspace until release, and then
only if there was no scroll events inbetween. This is closer to what
Xorg's wheel emulation does, and avoids spurious middle-click pastes.

Signed-off-by: Jamie Lentin <jm@lentin.co.uk>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>