ath9k: Reset caldata on radio enable

Not doing so, the caldata continues to retain older history
values learned on that channel. It is always safer to start
noise floor calibration from the defaults after the assoication.
So this patch resets the nf history buffer when none of the
STA vifs are associated.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>