HID: wacom: generic: Support multiple tools per report

Some Wacom devices contain contain Pen and Pad usages in
the same report. Future devices of this type may utilize
HID Descriptors.

The generic code path of the Wacom driver previously
assumed pen, touch, and pad reports were delivered in
separate reports. This patch adds support for processing
each collection of a report separately, in order to support
reports with multiple tools.

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>