net: dsa: mv88e6xxx: factorize in-chip bridge map

Factorize the code in the DSA port_bridge_{join,leave} routines used to
program the port VLAN map of all local ports of a given bridge group.

At the same time shorten the _mv88e6xxx_port_based_vlan_map to get rid
of the old underscore prefix naming convention.

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>