net: dsa: mv88e6xxx: error handling for serdes_power functions

Added default case to handle undefined cmode scenario in
mv88e6393x_serdes_power() and mv88e6393x_serdes_power() methods.

Addresses-Coverity: 1494644 ("Uninitialized scalar variable")
Fixes: 21635d9 (net: dsa: mv88e6xxx: Fix application of erratum 4.8 for 88E6393X)
Reviewed-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Ameer Hamza <amhamza.mgc@gmail.com>
Link: https://lore.kernel.org/r/20211209041552.9810-1-amhamza.mgc@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: amer-hamza

drivers/net/dsa/mv88e6xxx/serdes.c

@@ -830,7 +830,7 @@ int mv88e6390_serdes_power(struct mv88e6xxx_chip *chip, int port, int lane,
 			   bool up)
 {
 	u8 cmode = chip->ports[port].cmode;
-	int err = 0;
+	int err;
 
 	switch (cmode) {
 	case MV88E6XXX_PORT_STS_CMODE_SGMII:
@@ -842,6 +842,9 @@ int mv88e6390_serdes_power(struct mv88e6xxx_chip *chip, int port, int lane,
 	case MV88E6XXX_PORT_STS_CMODE_RXAUI:
 		err = mv88e6390_serdes_power_10g(chip, lane, up);
 		break;
+	default:
+		err = -EINVAL;
+		break;
 	}
 
 	if (!err && up)
@@ -1541,6 +1544,9 @@ int mv88e6393x_serdes_power(struct mv88e6xxx_chip *chip, int port, int lane,
 	case MV88E6393X_PORT_STS_CMODE_10GBASER:
 		err = mv88e6390_serdes_power_10g(chip, lane, on);
 		break;
+	default:
+		err = -EINVAL;
+		break;
 	}
 
 	if (err)