net: bonding: Share lacpdu_mcast_addr definition

There are already a few definitions of arrays containing
MULTICAST_LACPDU_ADDR and the next patch will add one more use. These all
contain the same constant data so define one common instance for all
bonding code.

Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>