net: ethernet: ti: cpsw: fix lost of mcast packets while rx_mode update

Whenever kernel or user decides to call rx mode update, it clears
every multicast entry from forwarding table and in some time adds
it again. This time can be enough to drop incoming multicast packets.

That's why clear only staled multicast entries and update or add new
one afterwards.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>