ukbd(4): Avoid races in LED setting on attach.

- Don't reuse sc_delay for LED task -- a keyboard interrupt shortly
  after attach might reset sc_delay so that the LEDs never get turned
  back off.

- Don't turn the LEDs back off after attach if something else has
  already changed them by the time the callout fires.

(And make sure to callout_halt before done this time!)

(riastradh)