net: bridge: update multicast stats from maybe_deliver()

Simplify this code by updating bridge multicast stats from
maybe_deliver().

Note that commit 6db6f0ea ("bridge: multicast to unicast"), in case
the port flag BR_MULTICAST_TO_UNICAST is set, never updates the previous
port pointer, therefore it is always going to be different from the
existing port in this deduplicated list iteration.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>