ath9k: port to cfg80211 rfkill

This ports the ath9k rfkill code to the new API offered by
cfg80211 and thus removes a lot of useless stuff.

("With this series a kernel panic, which is a regression, during module
unload disappears." -- Vasanthakumar Thiagarajan <vasanth@atheros.com>

Other patches in the series:

  ath9k: Add helper to get ath9k specific current channel
  ath9k: Make sure we have current channel in ah_curchan before rf
    disable/enable

-- JWL)

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Luis Rodriguez <mcgrof@gmail.com>
Tested-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>