net: dsa: Eliminate dsa_slave_get_link()

Since we use PHYLIB to manage the per-port link indication, this will
also be reflected correctly in the network device's carrier state, so we
can use ethtool_op_get_link() instead.

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