net: dsa: qca8k: add op to get ports netdev

In order that the LED trigger can blink the switch MAC ports LED, it
needs to know the netdev associated to the port. Add the callback to
return the struct device of the netdev.

Add an helper function qca8k_phy_to_port() to convert the phy back to
dsa_port index, as we reference LED port based on the internal PHY
index and needs to be converted back.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>