net: dsa: Pass device pointer to dsa_register_switch

In preparation for allowing dsa_register_switch() to be supplied with
device/platform data, pass down a struct device pointer instead of a
struct device_node.

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