dsa_port_link_register_of() and dsa_port_link_unregister_of() are not
written with the fact in mind that they can be called with a dp->dn that
is NULL (as evidenced even by the _of suffix in their name), but this is
exactly what happens.

How this behaves will differ depending on whether the backing driver
implements ->adjust_link() or not.

If it doesn't, the "if (of_phy_is_fixed_link(dp->dn) || phy_np)"
condition will return false, and dsa_port_link_register_of() will do
nothing and return 0.

If the driver does implement ->adjust_link(), the
"if (of_phy_is_fixed_link(dp->dn))" condition will return false
(dp->dn is NULL) and we will call dsa_port_setup_phy_of(). This will
call dsa_port_get_phy_device(), which will also return NULL, and we will
also do nothing and return 0.

It is hard to maintain this code and make future changes to it in this
state, so just suppress calls to these 2 functions if dp->dn is NULL.
The only functional effect is that if the driver does implement
->adjust_link(), we'll stop printing this to the console:

Using legacy PHYLIB callbacks. Please migrate to PHYLINK!

but instead we'll always print:

[    8.539848] dsa-loop fixed-0:1f: skipping link registration for CPU port 5

This is for the better anyway, since "using legacy phylib callbacks"
was misleading information - we weren't issuing _any_ callbacks due to
dsa_port_get_phy_device() returning NULL.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>