net: dsa: mv88e6xxx: mask apparently non-existing phys during probing

To avoid excessive mdio bus transactions during probing, mask all phy
addresses that do not exist (there is a 1:1 mapping between switch port
number and phy address).

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Klaus Kudielka <klaus.kudielka@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>