ipv6: suppress sparse warnings in IP6_ECN_set_ce()

Pass the correct type __wsum to csum_sub() and csum_add(). This doesn't
really change anything since __wsum really *is* __be32, but removes the
address space warnings from sparse.

Cc: Eric Dumazet <edumazet@google.com>
Fixes: 34ae6a1a

 ("ipv6: update skb->csum when CE mark is propagated")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>