HID: wacom: Consult the application usage when determining field type

It is not necessarily sufficient to look only at the physical and logical
usages when determining if a field is for the pen or touch. Some fields
are not contained in a sub-collection and thus only have an application
usage. Not checking the application usage in such cases causes us to
ignore the field entirely, which may lead to incorrect behavior.

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