fixed IP checksum calculation

Closes L2SDP-343

libraries/io/eth/src/vhdl/eth_checksum.vhd

@@ -150,7 +150,7 @@ BEGIN
                   sum;
   
   -- Accumulate the last carry
-  last_dat    <= sum(c_halfword_w-1 DOWNTO 0) + sum_cin;
+  last_dat    <= sum(c_halfword_w-1 DOWNTO 0) + sum_cin + word_sum_cin; -- Also add word_sum_cin in the case that the last word has a carry.
                     
   -- Checksum is 1-complement of the sum
   nxt_checksum     <= NOT(STD_LOGIC_VECTOR(last_dat)) WHEN prev_in_eop_dly='1' ELSE i_checksum;