macvlan: handle set_promiscuity failures

It's quite unlikely that dev_set_promiscuity will fail,
but worth checking just in case.

Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>