drivers: net: cpsw: fix cpsw hung with add vlan using vconfig

while adding vlan in dual EMAC mode, only specific ports should be
subscribed for the vlan, else it will lead to switching mode and
if both ports connected to same switch cpsw will hung as it creates
a network loop. Fixing this by adding only specific ports in case
of dual EMAC.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>