net: phy: don't allow __set_phy_supported to add unsupported modes

Currently __set_phy_supported allows to add modes w/o checking whether
the PHY supports them. This is wrong, it should never add modes but
only remove modes we don't want to support.

The commit marked as fixed didn't do anything wrong, it just copied
existing functionality to the helper which is being fixed now.

Fixes: f3a6bd39 ("phylib: Add phy_set_max_speed helper")
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>