ath5k: Use static const

Using static const generally increases object text and decreases data size.
It also generally decreases overall object size.

   text	   data	    bss	    dec	    hex	filename
  11266	     56	   2464	  13786	   35da	drivers/net/wireless/ath/ath5k/ani.o.old
  11181	     56	   2464	  13701	   3585	drivers/net/wireless/ath/ath5k/ani.o.new

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>