net: ixp4xx_hss: fix duplicate HDLC netdev allocation

In the Linux kernel, the following vulnerability has been resolved: net: ixp4xx_hss: fix duplicate HDLC netdev allocation ixp4xx_hss_probe() allocates two HDLC netdevs. The first one is stored in ndev, initialized, and registered with register_hdlc_device(). The second one is stored in port->netdev and later used by the remove path for unregister_hdlc_device() and free_netdev(). This means that the registered netdev is not the same object that is unregistered and freed on remove. It also leaks the first allocation if the second alloc_hdlcdev() call fails, and the first allocation is not checked before ndev is used. Older code allocated the HDLC netdev only once and stored the same object in both the local variable and port->netdev. The buggy conversion split this into two alloc_hdlcdev() calls. A later rename changed the local variable name to ndev, but the underlying mismatch remained. Fix this by allocating the HDLC netdev only once and assigning the same object to port->netdev.