ath5k: Add Spur filter support on newer chips

* Add spur filter support for RF5413 and later chips

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: Bob Copeland <me@bobcopeland.com>

Signed-off-by: John W. Linville <linville@tuxdriver.com>