This solves a bug with the wireless receiver:
- at plug, the wireless receiver does not know which Wacom device it is
  connected to, so it does not actually creates all the LEDs
- when the tablet connects, wacom->wacom_wac.features.type is set to the
  proper device so that wacom_wac can understand the packets
- when the receiver is unplugged, it detects that a LED should have been
  created (based on wacom->wacom_wac.features.type) and tries to remove
  it: crash when removing the sysfs group.

Side effect, we can now safely call several times wacom_destroy_leds().
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>