bridge: remove temporary variable for MLDv2 maximum response code computation

As suggested by Stephen Hemminger, this remove the temporary variable
introduced in commit eca2a43b
("bridge: fix icmpv6 endian bug and other sparse warnings")

Signed-off-by: Ang Way Chuang <wcang@sfc.wide.ad.jp>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>