pppoe: drop PPPOX_ZOMBIEs in pppoe_flush_dev

otherwise we loop forever if a PPPoE socket was set
to PPPOX_ZOMBIE state by a PADT message when the
ethernet device is going down afterwards.

Signed-off-by: Ulrich Weber <uweber@astaro.com>
Signed-off-by: David S. Miller <davem@davemloft.net>