drivers: net: cpsw: fix multicast flush in dual emac mode

Since ALE table is a common resource for both the interfaces in Dual EMAC
mode and while bringing up the second interface in cpsw_ndo_set_rx_mode()
all the multicast entries added by the first interface is flushed out and
only second interface multicast addresses are added. Fixing this by
flushing multicast addresses based on dual EMAC port vlans which will not
affect the other emac port multicast addresses.

Fixes: d9ba8f9e (driver: net: ethernet: cpsw: dual emac interface implementation)
Cc: <stable@vger.kernel.org> # v3.9+
Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>