carl9170: 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
   1897	     56	    672	   2625	    a41	drivers/net/wireless/ath/carl9170/cmd.o.new
   1897	     56	    672	   2625	    a41	drivers/net/wireless/ath/carl9170/cmd.o.old

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