HID: lenovo: Prepare support for adding other devices

Ensure all tpkbd specifics are within a postfixed function, the
main functions for the driver should just switch to the appropriate
function depending on product ID. Given this, we can add extra devices
by including extra postfixed functions.

Signed-off-by: Jamie Lentin <jm@lentin.co.uk>
Reviewed-by: Antonio Ospite <ao2@ao2.it>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>