i40e: Make struct i40e_stats const

Move some data to text

$ size drivers/net/ethernet/intel/i40e/i40e_ethtool.o*
   text	   data	    bss	    dec	    hex	filename
  25012	      0	     32	  25044	   61d4	drivers/net/ethernet/intel/i40e/i40e_ethtool.o.new
  22868	   2120	     32	  25020	   61bc	drivers/net/ethernet/intel/i40e/i40e_ethtool.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>