HID: wacom: use WACOM_*_FIELD macros in wacom_usage_mapping()

We introduced nice macros in wacom_wac.c to check whether a field is
a pen or a touch one.

wacom_usage_mapping() still uses it's own tests, which are not in sync with
the wacom_wac tests (.application is not checked).

That means that some legitimate fields might be filtered out from the
usage mapping, and thus will not be used properly while receiving the
events.

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