ath9k: classify DFS debug header further

DFS_DEBUG_H is very generic, instead use something more specific
to ath9k such as ATH9K_DFS_DEBUG_H.

Reported-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>