net: dsa: mv88e6xxx: read cmode on mv88e6320/21 serdes only ports

On the mv88e6320 and 6321 switch family, port 0/1 are serdes only ports.
Modified the mv88e6352_get_port4_serdes_cmode function to pass a port
number since the register set of the 6352 is equal on the 6320/21.

Signed-off-by: Steffen Bätz <steffen@innosonix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20240508072944.54880-3-steffen@innosonix.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>