drivers: net: cpsw: discard dual emac default vlan configuration

commit 02a54164c52ed6eca3089a0d402170fbf34d6cf5 upstream.

In Dual EMAC, the default VLANs are used to segregate Rx packets between
the ports, so adding the same default VLAN to the switch will affect the
normal packet transfers. So returning error on addition of dual EMAC
default VLANs.

Even if EMAC 0 default port VLAN is added to EMAC 1, it will lead to
break dual EMAC port separations.

Fixes: d9ba8f9e (driver: net: ethernet: cpsw: dual emac interface implementation)
Reported-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>