ath9k: make private driver rate tables const

On x86 this allows us to do the following small savings:
shave off 23 % off of the module's data, and
shave off 6  % off of the module's text.

We save 456 bytes, for those counting.

$ size ath9k.ko
   text    data     bss     dec     hex filename
 250794    3628    1600  256022   3e816 ath9k.ko
$ size ath9k-old.ko
   text    data     bss     dec     hex filename
 239114   15308    1600  256022   3e816 ath9k-old.ko

$ du -b ath9k.ko
4034244 ath9k.ko
$ du -b ath9k-old.ko
4033788 ath9k-old.ko

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath/ath9k/ath9k.h b/drivers/net/wireless/ath/ath9k/ath9k.h
index 90b6314..10ffc94 100644
--- a/drivers/net/wireless/ath/ath9k/ath9k.h
+++ b/drivers/net/wireless/ath/ath9k/ath9k.h
@@ -576,8 +576,8 @@
 	struct ath_tx tx;
 	struct ath_beacon beacon;
 	struct ieee80211_rate rates[IEEE80211_NUM_BANDS][ATH_RATE_MAX];
-	struct ath_rate_table *hw_rate_table[ATH9K_MODE_MAX];
-	struct ath_rate_table *cur_rate_table;
+	const struct ath_rate_table *hw_rate_table[ATH9K_MODE_MAX];
+	const struct ath_rate_table *cur_rate_table;
 	struct ieee80211_supported_band sbands[IEEE80211_NUM_BANDS];
 
 	struct ath_led radio_led;