libertas: 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
   3650	     56	    704	   4410	   113a	drivers/net/wireless/libertas/rx.o.new
   3695	     56	    704	   4455	   1167	drivers/net/wireless/libertas/rx.o.old
  27328	    964	   5240	  33532	   82fc	drivers/net/wireless/libertas/cfg.o.new
  27328	    964	   5240	  33532	   82fc	drivers/net/wireless/libertas/cfg.o.old

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