mlxsw: spectrum_switchdev: Reduce scope of bridge struct

Some attributes in the global chip struct are only relevant for bridge
operation, so encapsulate them in their own struct that isn't exposed to
non-bridge code.

This will also help us later, when we add more bridge-specific
attributes.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>