net: of_mdio: fix of_set_phy_supported after driver probing

Commit 8fdade4b

 ("net: of_mdio: parse "max-speed" property to set PHY
supported features") introduced a typo in of_set_phy_supported for the
first assignment of phydev->supported which will not effectively limit
the PHY device supported features bits if the PHY driver contains
"higher" features (e.g: max-speed = <100> and PHY driver has
PHY_GBIT_FEATURES set).

Fix this by making sure that the very first thing is to reset to sane
defaults (PHY_BASIC_FEATURES) and then progressively add speed features
as we parse them.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>