net: eth-uclass: Fix for DM USB ethernet support

When a USB ethernet device is halted, the device driver is removed. When
this happens the uclass private memory is freed and uclass_priv is set to
NULL. This causes a data abort when uclass_priv->state is then set to
ETH_STATE_PASSIVE.

Fix it by checking if uclass_priv is NULL before setting uclass_priv->state

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

net/eth-uclass.c

@@ -312,7 +312,8 @@ void eth_halt(void)
 
 	eth_get_ops(current)->stop(current);
 	priv = current->uclass_priv;
-	priv->state = ETH_STATE_PASSIVE;
+	if (priv)
+		priv->state = ETH_STATE_PASSIVE;
 }
 
 int eth_is_active(struct udevice *dev)