net: phy: phy_link_topology: Add a helper to retrieve ports

In order to allow netlink access to phy_ports, let's add a helper to
retrieve them. When handling a port coming from phy_link_topology, the
caller must hold rtnl until it's done with it.

We don't really support PHY and phy_port operation on ops-locked devices
yet, warn when we try to do so on such an interface.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Aleksei Sviridkin <f@lex.la>
Reviewed-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Tested-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20261016133107.54054-8-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>