switchdev: fix return value of switchdev_port_fdb_dump in case of error

switchdev_port_fdb_dump is used as .ndo_fdb_dump. Its return value is
idx, so we cannot return errval.

Fixes: 45d4122c

 ("switchdev: add support for fdb add/del/dump via switchdev_port_obj ops.")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Acked-by: Scott <Feldman&lt;sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>