HID: wacom: add support of the Pen of the Bamboo Pad

Bamboo Pads are using the generic processing but their report descriptors
differ from the ISDv* line. The pen fields are marked with the .physical
as Digitizer_Pen, which makes also sense.

Add this field to the checks and enable for free Bamboo Pads.

Reported-by: Josep Sanchez Ferreres <josep.sanchez.ferreres@est.fib.upc.edu>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>