net: dsa: convert dsa_user_phylink_fixed_state() to use dsa_phylink_to_port()

Convert dsa_user_phylink_fixed_state() to use the newly introduced
dsa_phylink_to_port() helper.

Suggested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>