net: dsa: mv88e6xxx: Fixed port netdev check for VLANs

Having the same VLAN on multiple bridges is currently unsupported as
an offload. mv88e6xxx_port_check_hw_vlan() is used to ensure that a
VLAN is not on multiple bridges when adding a VLAN range to a port. It
loops the ports and checks to see if there are ports in a different
bridge with the same VLAN.

While walking all switch ports, the code was checking if the new port
has a netdev slave attached to it. If not, skip checking the port
being walked. This seems like a typ0. If the new port does not have a
slave, how has a VLAN been added to it in the first place, requiring
this check be performed at all? More likely, we should be checking if
the port being walked has a slave. Without the port having a slave, it
cannot have a VLAN on it, so there is no need to check further for
that particular port.

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