ath9k: change DFS logging to use ath_dbg()

The DFS pattern detector was initially planned to reside on
a higher layer and used generic pr_*() logging functions.

Being part of ath9k, use ath_dbg() instead and make DFS log
ouput selectable via ATH_DBG_DFS (0x20000) at runtime.

This patch does not contain functional modifications.

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>