P2P: Postpone P2P scan only if station mode scan is pending

P2P: Postpone P2P scan only if station mode scan is pending

sta_scan_pending was supposed to be accurate enough for determining
whether a P2P scan needs to be postponed. However, it has turned out
that there were cases where sta_scan_pending was not cleared properly.
While the known cases have now been addressed, it is possible that some
other cases may still exist. To avoid issues with P2P operationg getting
stuck, verify more carefully that there is a real pending station mode
scan (either in progress or scheduled to be requested).

Signed-hostap: Jouni Malinen <jouni@qca.qualcomm.com>