HID: wacom: generic: add the "Report Valid" usage

Wacom Bluetooth reports contain multiple pen frames per report.
Each frame contains a flag indicating if the frame is valid.
Future Wacom devices with this type of report may contain HID
descriptors, add support for this usage to the generic codepath
of the Wacom driver.

Signed-off-by: Aaron Armstrong Skomra <skomra@gmail.com>
Reviewed-by: Ping Cheng <ping.cheng@wacom.com>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>