bridge: Fix IGMP3 report parsing

The IGMP3 report parsing is looking at the wrong address for
group records.  This patch fixes it.

Reported-by: Banyeer <banyeer@yahoo.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>