ath9k: dfs move ath_dfs_pool_stats

Move ath_dfs_pool_stats to dfs_pattern_detector
code to be not specyfic only for ath9k.

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>