net: dsa: microchip: ksz8795: Fix KSZ8794 port map again

The KSZ8795 switch has 4 external ports {0,1,2,3} and 1 CPU port {4}, so
does the KSZ8765. The KSZ8794 seems to be repackaged KSZ8795 with different
ID and port 3 not routed out, however the port 3 registers are present in
the silicon, so the KSZ8794 switch has 3 external ports {0,1,2} and 1 CPU
port {4}. Currently the driver always uses the last port as CPU port, on
KSZ8795/KSZ8765 that is port 4 and that is OK, but on KSZ8794 that is port
3 and that is not OK, as it must also be port 4.

This patch adjusts the driver such that it always registers a switch with
5 ports total (4 external ports, 1 CPU port), always sets the CPU port to
switch port 4, and then configures the external port mask according to
the switch model -- 3 ports for KSZ8794 and 4 for KSZ8795/KSZ8765.

Fixes: 68a1b676 ("net: dsa: microchip: ksz8795: remove superfluous port_cnt assignment")
Fixes: 4ce2a984 ("net: dsa: microchip: ksz8795: use phy_port_cnt where possible")
Fixes: 241ed719

 ("net: dsa: microchip: ksz8795: use port_cnt instead of TOTOAL_PORT_NUM")
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Michael Grzeschik <m.grzeschik@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20210120001045.488506-1-marex@denx.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>