net: dsa: mv88e6xxx: fix 88E6393X family internal phys layout

88E6393X/88E6193X/88E6191X switches have in fact 8 internal PHYs, but those
are not present starting at port 0: supported ports go from 1 to 8

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>