net: dsa: utilize of_find_net_device_by_node

Using of_find_device_by_node() restricts the search to platform_device that
match the specified device_node pointer. This is not even remotely true for
network devices backed by a pci_device for instance.

of_find_net_device_by_node() allows us to do a more thorough lookup to find the
struct net_device corresponding to a particular device_node pointer.

For symetry with the non-OF code path, we hold the net_device pointer in
dsa_probe() just like what dev_to_net_dev() does when we call this
function.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>