ath9k: dfs kill ath9k specyfic code

Kill of using ath9k_hw_common() function
in dfs detector code.

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Reviewed-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>