net: dsa: mv88e6xxx: add POT operation

Add a pot_clear operation to clear the Priority Override Table and wrap
its call into a mv88e6xxx_pot_setup helper.

This allows us to get rid of the MV88E6XXX_FLAG_G2_POT flag.

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