net: dsa: rename dsa_port_link_{,un}register_of

There is a subset of functions that applies only to shared (DSA and CPU)
ports, yet this is difficult to comprehend by looking at their code alone.
These are dsa_port_link_register_of(), dsa_port_link_unregister_of(),
and the functions that only these 2 call.

Rename this class of functions to dsa_shared_port_* to make this fact
more evident, even if this goes against the apparent convention that
function names in port.c must start with dsa_port_.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>