net: dsa: mv88e6xxx: Handle error in serdes_get_regs

If the underlying read operation failed we would end up writing stale
data to the supplied buffer. This would end up with the last
successfully read value repeating. Fix this by only writing the data
when we know the read was good. This will mean that failed values will
return 0xffff.
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/mv88e6xxx/serdes.c

@@ -400,14 +400,16 @@ void mv88e6352_serdes_get_regs(struct mv88e6xxx_chip *chip, int port, void *_p)
 {
 	u16 *p = _p;
 	u16 reg;
+	int err;
 	int i;
 
 	if (!mv88e6352_port_has_serdes(chip, port))
 		return;
 
 	for (i = 0 ; i < 32; i++) {
-		mv88e6352_serdes_read(chip, i, &reg);
-		p[i] = reg;
+		err = mv88e6352_serdes_read(chip, i, &reg);
+		if (!err)
+			p[i] = reg;
 	}
 }
 
@@ -1096,6 +1098,7 @@ void mv88e6390_serdes_get_regs(struct mv88e6xxx_chip *chip, int port, void *_p)
 	u16 *p = _p;
 	int lane;
 	u16 reg;
+	int err;
 	int i;
 
 	lane = mv88e6xxx_serdes_get_lane(chip, port);
@@ -1103,8 +1106,9 @@ void mv88e6390_serdes_get_regs(struct mv88e6xxx_chip *chip, int port, void *_p)
 		return;
 
 	for (i = 0 ; i < ARRAY_SIZE(mv88e6390_serdes_regs); i++) {
-		mv88e6390_serdes_read(chip, lane, MDIO_MMD_PHYXS,
-				      mv88e6390_serdes_regs[i], &reg);
-		p[i] = reg;
+		err = mv88e6390_serdes_read(chip, lane, MDIO_MMD_PHYXS,
+					    mv88e6390_serdes_regs[i], &reg);
+		if (!err)
+			p[i] = reg;
 	}
 }