net: dsa: mv88e6xxx: Fix failed probe due to unsupported C45 reads

Not all mv88e6xxx device support C45 read/write operations. Those
which do not return -EOPNOTSUPP. However, when phylib scans the bus,
it considers this fatal, and the probe of the MDIO bus fails, which in
term causes the mv88e6xxx probe as a whole to fail.

When there is no device on the bus for a given address, the pull up
resistor on the data line results in the read returning 0xffff. The
phylib core code understands this when scanning for devices on the
bus. C45 allows multiple devices to be supported at one address, so
phylib will perform a few reads at each address, so although thought
not the most efficient solution, it is a way to avoid fatal
errors. Make use of this as a minimal fix for stable to fix the
probing problems.

Follow up patches will rework how C45 operates to make it similar to
C22 which considers -ENODEV as a none-fatal, and swap mv88e6xxx to
using this.

Cc: stable@vger.kernel.org
Fixes: 743a19e3 ("net: dsa: mv88e6xxx: Separate C22 and C45 transactions")
Reported-by: Tim Menninger <tmenninger@purestorage.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20240129224948.1531452-1-andrew@lunn.chSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/mv88e6xxx/chip.c

@@ -3659,7 +3659,7 @@ static int mv88e6xxx_mdio_read_c45(struct mii_bus *bus, int phy, int devad,
 	int err;
 
 	if (!chip->info->ops->phy_read_c45)
-		return -EOPNOTSUPP;
+		return 0xffff;
 
 	mv88e6xxx_reg_lock(chip);
 	err = chip->info->ops->phy_read_c45(chip, bus, phy, devad, reg, &val);