[PATCH net-next] ath9k: Use static const

Trivially reduces text size too. $ size drivers/net/wireless/ath/ath9k/debug.o* textdata bss dec hex filename 3443625285128 42092a46c drivers/net/wireless/ath/ath9k/debug.o.new 3446425285128 42120a488 drivers/net/wireless/ath/ath9k/debug.o.old

[PATCH net-next] ath9k: Use static const

Trivially reduces text size too. $ size drivers/net/wireless/ath/ath9k/debug.o* textdata bss dec hex filename 3443625285128 42092a46c drivers/net/wireless/ath/ath9k/debug.o.new 3446425285128 42120a488 drivers/net/wireless/ath/ath9k/debug.o.old