net: dsa: assign default CPU port to all ports

The current code only assigns the default cpu_dp to all user ports of
the switch to which the CPU port belongs. The user ports of the other
switches of the fabric thus don't have a default CPU port.

This patch fixes this by assigning the cpu_dp of all user ports of all
switches of the fabric when the tree is fully parsed.

Fixes: a29342e7

 ("net: dsa: Associate slave network device with CPU port")
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>