bridge: multicast: call skb_checksum_{simple_, }validate

Let's use these new, neat helpers.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>