net: dsa: mv88e6xxx: fix usable ports on 88e6020

net: dsa: mv88e6xxx: fix usable ports on 88e6020

The switch has 4 ports with 2 internal PHYs, but ports are numbered up
to 6, with ports 0, 1, 5 and 6 being usable.

Fixes: 71d94a432a15 ("net: dsa: mv88e6xxx: add support for MV88E6020 switch")
Signed-off-by: Michael Krummsdorf <michael.krummsdorf@tq-group.com>
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240326123655.40666-1-matthias.schiffer@ew.tq-group.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>