HID: wacom: generic: support the 'report valid' usage for touch

Finger data is separated into chunks in our Bluetooth report,
where each report contains the same number of chunks. Those chunks
are not aligned in any particular way to a set of finger touches.
That is, the first half of a group of simultaneous touches may
be in one chunk at the end of a report and the second half could
be at the beginning of the next report.

Also some chunks contain no data and potentially some chunks could
contain leftover (bad) data.

Introduce and process the WACOM_HID_WT_REPORT_VALID usage that the
device uses to let us know if we should process a chunk of data.

Signed-off-by: Aaron Armstrong Skomra <aaron.skomra@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>