rt2x00: 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
  40197	     56	   8336	  48589	   bdcd	drivers/net/wireless/rt2x00/rt2800lib.o.new
  40205	     56	   8336	  48597	   bdd5	drivers/net/wireless/rt2x00/rt2800lib.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>