net/mlx5: Bridge, fix uninitialized variable usage

In some conditions variable 'err' is not assigned with value in
mlx5_esw_bridge_port_obj_attr_set() and mlx5_esw_bridge_port_changeupper()
functions after recent changes to support LAG. Initialize the variable with
zero value in both cases.

Reported-by: Colin King <colin.king@canonical.com>
Reported-by: Tim Gardner <tim.gardner@canonical.com>
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
CC: linux-kernel@vger.kernel.org
Fixes: ff9b7521 ("net/mlx5: Bridge, support LAG")
Signed-off-by: Vlad Buslov <vladbu@nvidia.com>