net: dsa: mv88e6xxx: fix uninitialized error return

net: dsa: mv88e6xxx: fix uninitialized error return

The error return err is not initialized and there is a possibility
that err is not assigned causing mv88e6xxx_port_bridge_join to
return a garbage error return status. Fix this by initializing err
to 0.

Signed-off-by: Colin Ian King <[email protected]>
Reviewed-by: Vivien Didelot <[email protected]>
Signed-off-by: David S. Miller <[email protected]>