HID: wacom: Update vendor-defined usage names to better match standards

Our loose use of "pen" and "digitizer" in the naming of several of our
vendor-defined usages may be a source of confusion given that the terms
have specific meaning within the HID specification. "Pen" specifically
refers to "an integrated display that allows the use of a stylus" (e.g.
something like a tablet PC or Cintiq) wheras "Digitizer" is a better
fit for opaque tablets like an Intuos.

While we're at it, go ahead and rename the definitions to make them more
distinct and better match up with the convention used by HID (e.g. the use
of '_UP_' for usage pages) and make them more distinct.

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