ath9k: Use static const

Trivially reduces text size too.

$ size drivers/net/wireless/ath/ath9k/debug.o*
   text	   data	    bss	    dec	    hex	filename
  34436	   2528	   5128	  42092	   a46c	drivers/net/wireless/ath/ath9k/debug.o.new
  34464	   2528	   5128	  42120	   a488	drivers/net/wireless/ath/ath9k/debug.o.old

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