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()

and ieee80211_scan_done();

• remove corresponding workaround from rsu_join_bss().

Now the driver can associate to an AP with hidden SSID.

Tested with Asus USB-N10.