devlink: remove no longer true locking comment from port_new/del()

All commands are called holding instance lock. Remove the outdated
comment that says otherwise.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>