mlxsw: use devlink helper to generate physical port name

mlxsw: use devlink helper to generate physical port name

Since devlink knows the info needed to generate the physical port name
in a generic way for all devlink users, use the helper to do the job.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>