net: dsa: allow deferred probing

In preparation for allowing a different model to register DSA switches,
update dsa_of_probe() and dsa_probe() to return -EPROBE_DEFER where
appropriate.

Failure to find a phandle or Device Tree property is still fatal, but
looking up the internal device structure associated with a Device Tree
node is something that might need to be delayed based on driver probe
ordering.

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