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/hw.h b/drivers/net/wireless/ath/ath9k/hw.h
index ab34126..ddb24c4 100644
--- a/drivers/net/wireless/ath/ath9k/hw.h
+++ b/drivers/net/wireless/ath/ath9k/hw.h
@@ -580,7 +580,8 @@
 bool ath9k_hw_wait(struct ath_hw *ah, u32 reg, u32 mask, u32 val, u32 timeout);
 u32 ath9k_hw_reverse_bits(u32 val, u32 n);
 bool ath9k_get_channel_edges(struct ath_hw *ah, u16 flags, u16 *low, u16 *high);
-u16 ath9k_hw_computetxtime(struct ath_hw *ah, struct ath_rate_table *rates,
+u16 ath9k_hw_computetxtime(struct ath_hw *ah,
+			   const struct ath_rate_table *rates,
 			   u32 frameLen, u16 rateix, bool shortPreamble);
 void ath9k_hw_get_channel_centers(struct ath_hw *ah,
 				  struct ath9k_channel *chan,