HID: wacom: generic: support generic touch switch

The second generation Intuos Pro is the first device in the generic codepath
which has a touchswitch. We utilize a flag in wacom_shared in order to report
this switch event received from the pad on the touch input.

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