ar7: diag.sh: use common status_led_* functions

Switch to the common LED related status_led_blink_* functions in
set_state() as it's done in most of other targets.

Also, use "status_led_on" instead of "status_led_set_heartbeat"
and incorrect "status_led_set_on" in the "done" state.

Moreover, remove outdated comment from the script.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>