mac802.11: only wait for nl ack if unfinished

Jon reported that the setup via netlink of a raw 802.11 mon
device fails with:

  root~/gencfg# trafgen --in beacon.cfg --rfraw --out wlan0
  Waiting for netlink ack failed!

Let us only wait for it, if really needed.

Reported-by: Jon Schipp <jonschipp@gmail.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>