According to https://msdn.microsoft.com/en-us/library/windows/hardware/mt604195(v=vs.85).aspx
external buttons have some weird usage mapping:
- Button 2 Indicates Button State for external button for primary
  (default left) clicking.
- Button 3 Indicates Button State for external button for secondary
  (default right) clicking.

So in the current state, the buttons are mapped to right and middle.
Move the usage by one to correctly map the external buttons.
Tested-by: NChris Chiu <chiu@endlessm.com>
Signed-off-by: NBenjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>