net: lantiq: fix missing free_netdev() on error in ltq_etop_probe()

Add the missing free_netdev() before return from ltq_etop_probe()
in the error handling case.

Fixes: 14d4e308 ("net: lantiq: configure the burst length in ethernet drivers")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>