INPUT_PROP_BUTTONPAD is currently only set through the platform data.
The RMI4 header doc says that this property is there to force the
buttonpad property, so we also need to detect it by looking at
the exported buttons count.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reported-and-tested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>