ps2: fix mouse mappings for right/middle button

Commit 8b0caab0 ("ps2: add support for mice with extra/side buttons")
accidentally swapped right and middle mouse buttons. This commit corrects
the mapping as expected by the ps2 controller.

Signed-off-by: Fabian Lesniak <fabian@lesniak-it.de>
Message-id: 20170204150319.8907-1-fabian@lesniak-it.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>