iwlwifi: 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
  48644	     57	  12120	  60821	   ed95	drivers/net/wireless/b43/phy_n.o.new
  48661	     57	  12120	  60838	   eda6	drivers/net/wireless/b43/phy_n.o.old
  37906	     86	   7904	  45896	   b348	drivers/net/wireless/iwlwifi/iwl-agn-lib.o.new
  37937	     86	   7904	  45927	   b367	drivers/net/wireless/iwlwifi/iwl-agn-lib.o.old
  37781	    523	   6752	  45056	   b000	drivers/net/wireless/iwlwifi/iwl-3945.o.new
  37781	    523	   6752	  45056	   b000	drivers/net/wireless/iwlwifi/iwl-3945.o.old

Changed b43_nphy_write_clip_detection to take a const u16 *

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