Input: elantech - report the middle button of the touchpad

In the past, no elantech was known with 3 touchpad mouse buttons.
Fujitsu H730 is the first known elantech with a middle button. This commit
enables this middle button. For backwards compatibility, the Fujitsu is
detected via DMI, and only for this one 3 buttons will be announced.

Reported-by: Stefan Valouch <stefan@valouch.com>
Signed-off-by: Ulrik De Bie <ulrik.debie-os@e2big.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>