ath9k: Set ATH9K_STATION_STATISTICS when enabling debugging

The ATH9K_STATION_STATISTICS kernel config variable enables some extra
statistics that are useful for debugging (in particular with the airtime
fairness patches enabled). This adds that kernel config when selecting
ath9k debugging.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>