cfg80211: add flags to cfg80211_rx_mgmt()

Add flags intended to report various auxiliary information
and introduce the NL80211_RXMGMT_FLAG_ANSWERED flag to report
that the frame was already answered by the device.

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
[REPLIED->ANSWERED, reword commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>