commit aa972409 upstream.

Unfortunately, ForcePad capability is not actually exported over PS/2, so
we have to resort to DMI checks.
Reported-by: Nicole Faerber <nicole.faerber@kernelconcepts.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>