ath9k: Fix RFKILL bugs

This patch fixes 2 issues in RFKILL:

* Calling wiphy_rfkill_stop_polling() in ath9k_stop
  would mean that the driver cannot report HW status
  when the radio is re-enabled. Move this to ath_detach().

* Calling ath_radio_{enable/disable} without checking the current
  state results in ath_radio_enable() being called repeatedly
  for every invocation of rfkill_poll(). This is not needed
  in any case since wiphy_rfkill_set_hw_state() would call
  ->stop() if the radio has been disabled.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>