HID: wacom: Add support for Pro Pen slim

Wacom has introduced a new pen compatible with its MobileStudio Pro and
other tablets. Although adding it to the tool ID tablet is not strictly
necessary  unrecognized pens are reported as BTN_TOOL_PEN already, unless
the tablet sends the "eraser" bit, when BTN_TOOL_RUBBER is used instead),
we'll keep it updated anyway.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>