mlx5: ensure 0 is returned when vport is zero

Currently, if vport is zero then then an uninialized return status
in err is returned.  Since the only return status at the end of the
function esw_add_uc_addr is zero for the current set of return paths
we may as well just return 0 rather than err to fix this issue.

Detected by CoverityScan, CID#1452698 ("Uninitialized scalar variable")

Fixes: eeb66cdb ("net/mlx5: Separate between E-Switch and MPFS")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>