Input: wacom - TPC2FG doesn't store touch id for slots

Signed-off-by: Ping Cheng <pingc@wacom.com>
Tested-by: Rafi Rubin <rafi@seas.upenn.edu>
Reviewed-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>