HID: wacom: fix Bamboo ONE oops

Looks like recent changes in the Wacom driver made the Bamboo ONE crashes.
The tablet behaves as if it was a regular Bamboo device with pen, touch
and pad, but there is no physical pad connected to it.
The weird part is that the pad is still sending events and given that
there is no input node connected to it, we get  anull pointer exception.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1317116



Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Ping Cheng <pingc@wacom.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>