ath9k: make private driver rate tables const

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>