ath9k/ath: move dfs pattern detector to ath

Move the DFS pattern detector code to the ath module so
the other Atheros drivers can make us of it. This makes
no functional changes.

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>