net: lapbether: fix double free

The function netdev_priv() returns the private data of the device. The
memory to store the private data is allocated in alloc_netdev() and is
released in netdev_free(). Calling kfree() on the return value of
netdev_priv() after netdev_free() results in a double free bug.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: David S. Miller <davem@davemloft.net>