HID: wacom: store the hid_device pointers of the sibling devices

The Bamboo PAD in debug mode needs to re-route events from the debug
interface to the Pen interface. This can be easily done with
hid_input_report(), but that means that we need to keep a reference to
the various hid_devices.

There should be only one touch and one pen interface per physical tablet,
so there is no need to keep a list of hid-devices, plain pointers are
sufficient.

Tested-by: Josep Sanchez Ferreres <josep.sanchez.ferreres@est.fib.upc.edu>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>