atmel: 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
  42578	    720	   8528	  51826	   ca72	drivers/net/wireless/atmel.o.old
  42578	    720	   8528	  51826	   ca72	drivers/net/wireless/atmel.o.new

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