Input: wacom - share pen info with touch of the same ID

Touch enbaled devices share the same product ID with pen. However,
we do not want to post touch events while pen is in prox. To do so,
we used to keep a local static variable to keep track of if pen is
in prox or not. This works fine for Tablet PC devices since there
is only one device attached. With the newer touch enabled regular
tablets, we can not make this assumption any more, i.e, one system
may have more than one identical tablet plugged in.

This patch adds an new entry, shared, into the struct wacom_wac so
touch data can access pen data to locally. This solution assumes
the two tools (touch and pen) of the same ID will be probed one
after the other without interruption in between by another Wacom
device of the same ID.