net: dsa: mv88e6xxx: fix "don't use PHY_DETECT on internal PHY's"

This commit fixes a misunderstanding in commit 4a3e0aed ("net: dsa:
mv88e6xxx: don't use PHY_DETECT on internal PHY's").

For Marvell DSA switches with the PHY_DETECT bit (for non-6250 family
devices), controls whether the PPU polls the PHY to retrieve the link,
speed, duplex and pause status to update the port configuration. This
applies for both internal and external PHYs.

For some switches such as 88E6352 and 88E6390X, PHY_DETECT has an
additional function of enabling auto-media mode between the internal
PHY and SERDES blocks depending on which first gains link.

The original intention of commit 5d5b231d (net: dsa: mv88e6xxx: use
PHY_DETECT in mac_link_up/mac_link_down) was to allow this bit to be
used to detect when this propagation is enabled, and allow software to
update the port configuration. This has found to be necessary for some
switches which do not automatically propagate status from the SERDES to
the port, which includes the 88E6390. However, commit 4a3e0aed
("net: dsa: mv88e6xxx: don't use PHY_DETECT on internal PHY's") breaks
this assumption.

Maarten Zanders has confirmed that the issue he was addressing was for
an 88E6250 switch, which does not have a PHY_DETECT bit in bit 12, but
instead a link status bit. Therefore, mv88e6xxx_port_ppu_updates() does
not report correctly.

This patch resolves the above issues by reverting Maarten's change and
instead making mv88e6xxx_port_ppu_updates() indicate whether the port
is internal for the 88E6250 family of switches.

  Yes, you're right, I'm targeting the 6250 family. And yes, your
  suggestion would solve my case and is a better implementation for
  the other devices (as far as I can see).

Fixes: 4a3e0aed

 ("net: dsa: mv88e6xxx: don't use PHY_DETECT on internal PHY's")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Tested-by: Maarten Zanders <maarten.zanders@mind.be>
Link: https://lore.kernel.org/r/E1muXm7-00EwJB-7n@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>