mlxsw: spectrum_switchdev: Save MAC and FID as a key in 'struct mlxsw_sp_mdb_entry'

The next patch will add support for storing all the MDB entries in a hash
table. As a preparation, save the MAC address and the FID in a
separate structure. This structure will be used later as a key for the
hash table.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>