mac80211: don't flush when probing the AP

All the callers of ieee80211_mgd_probe_ap_send return right
after they call the flush() callback. This means that calling
flush() is uneeded since its meaning is to wait until the
queues of the device are empty.

Devices that know how to report status on Tx will do so using
the regular path (ieee80211_tx_status) and this status will
trigger the continuation of the flow of the probe
(ieee80211_sta_tx_notify).

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>