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 <colin.king@canonical.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>