net: dsa: mv88e6xxx: rework jumbo size operation

Marvell chips have a Jumbo Mode to set the maximum frame size (MTU).

The mv88e6xxx_ops structure is meant to contain generic functionalities,
no driver logic. Change port_jumbo_config to port_set_jumbo_size setting
the mode from a given maximum size value.

There is no functional changes since we still use 10240 bytes.

At the same time, correctly clear all Jumbo Mode bits before writing.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>