dsa_to_port function in 5.10 returns dsa_port from the port list in
dsa_switch_tree, but the tree is built when the switch is registered
by dsa_register_switch and it's null in rtl83xx_mdio_probe.
So, we need to use dsa_to_port after the registration of the switch.
Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>