HID: wacom: Report input events for each finger on generic devices

The existing generic touch code only reports events after reading an
entire HID report, which practically means that only data about the last
contact in a report will ever be provided to userspace. This patch uses
a trick from hid-multitouch.c to discover what type of field is at the
end of each contact; when such a field is encountered all the stored
contact data will be reported.

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