b43: 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
   5502	     56	   1336	   6894	   1aee	drivers/net/wireless/b43/phy_common.o.new
   5511	     56	   1336	   6903	   1af7	drivers/net/wireless/b43/phy_common.o.old

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