Input: wacom - mark Intuos5 pad as in-prox when touching buttons

If the ExpressKeys on the Intuos5 are touched, they currently result
an out-of-prox packet being sent even if the pad is already out of
prox. This can cause some confusion in the X driver. To restore the
expected semantics, we make being touched a sufficient condition to
signal proximity.

https://bugs.freedesktop.org/show_bug.cgi?id=54250

Reported-by: Timo Aaltonen <tjaalton@ubuntu.com>
Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Reviewed-by: Chris Bagwell <chris@cnpbagwell.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>