of: mdio: fix compile warning in of_mdiobus_register_phy()

Commit de906af1 (net: phy: make of_set_phy_supported work with genphy driver)
removed the last user of variable 'max_speed' in function
of_mdiobus_register_phy(), leading to compile warning "unused variable
‘max_speed’ [-Wunused-variable]". Thus remove it.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>