broadcom: b44: prevent uninitialized value usage

On execution path with raised B44_FLAG_EXTERNAL_PHY, b44_readphy()
leaves bmcr value uninitialized and it is used later in the code.

Add check of this flag at the beginning of the b44_nway_reset() and
exit early of the function with restarting autonegotiation if an
external PHY is used.

Fixes: 753f492 ("[B44]: port to native ssb support")
Reviewed-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Alexey Simakov <bigalex934@gmail.com>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Link: https://patch.msgid.link/20251205155815.4348-1-bigalex934@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: jtstrs

drivers/net/ethernet/broadcom/b44.c

@@ -1790,6 +1790,9 @@ static int b44_nway_reset(struct net_device *dev)
 	u32 bmcr;
 	int r;
 
+	if (bp->flags & B44_FLAG_EXTERNAL_PHY)
+		return phy_ethtool_nway_reset(dev);
+
 	spin_lock_irq(&bp->lock);
 	b44_readphy(bp, MII_BMCR, &bmcr);
 	b44_readphy(bp, MII_BMCR, &bmcr);