net: bridge: convert mcast options to bits

This patch converts the rest of the mcast options to bits. It also packs
the mcast options a little better by moving multicast_mld_version to an
existing hole, reducing the net_bridge size by 8 bytes.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>