bridge: mcast: Constify 'group' argument in br_multicast_new_port_group()

The 'group' argument is not modified, so mark it as 'const'. It will
allow us to constify arguments of the callers of this function in future
patches.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>