HID: wacom: Declare tool ID 0x84a as an Intuos eraser

The eraser end of the 8K pen available for the MobileStudio Pro has a tool
ID of 0x84a. The 'wacom_intuos_get_tool_type' function does not currently
recognize this ID, causing it to return BTN_TOOL_PEN rather than
BTN_TOOL_RUBBER. This does not cause a problem for the MobileStudio Pro
since, as a HID_GENERIC device, the driver relies on the state of the
HID_DG_INVERT usage instead. It would, however, cause problems if the pen
is used with devices that use the traditional 'wacom_intuos_irq' codepath
instead.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Acked-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>