Input: mt - update input_mt_report_slot_state() kerneldoc

Commit c258e84b180d ("Input: do not assign new tracking ID when changing
tool type") stopped assigning a new tracking ID when the tool type in a
slot changes, but left the kerneldoc comment for
input_mt_report_slot_state() stating the old behavior. Update the
comment to match the implementation.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>