drivers: net: cpsw: fix dual EMAC stall when connected to same switch

In commit 629c9a8f (drivers: net: cpsw: Add
default vlan for dual emac case also), api cpsw_add_default_vlan() also
changes the port vlan which is required to seperate the ports which results
in the following behavior

In Dual EMAC mode, when both the Etnernet connected is connected to same
switch, it creates a loop in the switch and when a broadcast packet is
received it is forwarded to the other port which stalls the whole switch
and needs a reset/power cycle to the switch to recover. So intead of using
the api, add only the default VLAN entry in dual EMAC case.

Cc: Yegor Yefremov <yegorslists@googlemail.com>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Tested-by: Yegor Yefremov <yegorslists@googlemail.com>
Tested-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>