net: phy: fix reading fixed phy status

With the switch to phy_resolve_aneg_linkmode() we don't read from the
chip any longer what is advertised but use phydev->advertising directly.
For a fixed phy however this bitmap is empty so far, what results in
no common mode being found. This breaks DSA. Fix this by advertising
everything that is supported. For a normal phy this done by phy_probe().

Fixes: 5502b218

 ("net: phy: use phy_resolve_aneg_linkmode in genphy_read_status")
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>