net: dsa: mv88e6xxx: fix 88E6321 family comment

MV88E6XXX_FAMILY_6321 is undefined, 88E6321's family is 88E6320,
fix this.

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>