rt2x00: Fix rfkill_polling register function.

  Move rt2x00rfkill_register(rt2x00dev) to rt2x00lib_probe_dev
  function. It fixes of starting rfkill_poll function at the
  right time if sets hard rfkill block and reboot. rt2x00mac_rfkill_poll
  should be starting before bringing up the wireless interface.

Signed-off-by: Chen, Chien-Chia <machen@suse.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
CC: Kevin Chou <kevin.chou@mediatek.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>