rsu: various scanning fixes.
- Set IEEE80211_FEXT_SCAN_OFFLOAD flag; firmware can send null data
frames when associated.
- Check IEEE80211_SCAN_ACTIVE scan flag instead of IEEE80211_F_ASCAN
ic flag; the last is never set since r170530.
- Eliminate software scan (net80211) <-> site_survey (driver) race:
- override ic_scan_curchan and ic_scan_mindwell pointers so net80211
will not try to finish scanning automatically;
- inform net80211 about current status via ieee80211_cancel_scan()
- remove corresponding workaround from rsu_join_bss().
Now the driver can associate to an AP with hidden SSID.
Tested with Asus USB-N10.