net: dsa: Simplify dsa_slave_phy_setup()

Remove the code that tried to identify if a PHY designated by Device
Tree required diversion through the DSA-created MDIO bus. This was
created mainly for the bcm_sf2.c driver back when it did not have its
own MDIO bus driver, which it now has since 461cd1b0

 ("net: dsa:
bcm_sf2: Register our slave MDIO bus").

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Martin Hundebøll <mnhu@prevas.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>