pppoe: uninitialized variable in pppoe_flush_dev()

This assignment got deleted along with the checks by mistake.  This
comes from:  8753d29f "pppoe: remove unnecessary checks in
pppoe_flush_dev"

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>