net: dsa: Make most functions take a dsa_port argument

In preparation for allowing platform data, and therefore no valid
device_node pointer, make most DSA functions takes a pointer to a
dsa_port structure whenever possible. While at it, introduce a
dsa_port_is_valid() helper function which checks whether port->dn is
NULL or not at the moment.

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