HID: magicmouse: fix 3 button emulation of Mouse 2

It is observed that, with 3 button emulation, when middle button is
clicked, either the left button or right button is clicked as well. It
is caused by hidinput "correctly" acting on the event, oblivious to the
3 button emulation.

As raw_event has taken care of everything, no further processing is
needed. However, the only way to stop at raw_event is to return an error
(negative) value. Therefore, the processing is stopped at event instead.

[jkosina@suse.cz: fix comment style]
Signed-off-by: John Chen <johnchen902@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>