devlink: use xa_for_each_start() helper in devlink_nl_cmd_port_get_dump_one()

As xarray has an iterator helper that allows to start from specified
index, use this directly and avoid repeated iteration from 0.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>