Clean up HT40 scan and share nl80211 scanning code

Clean up HT40 scan and share nl80211 scanning code

Instead of adding a new driver_ops for fetching neighbor BSS data (that
nl80211 driver interface had to scan during initialization), share the
same scan operations that wpa_supplicant is using. This gets rid of
duplicated scan code in driver_nl80211.c (and better yet, removes large
part of old WEXT code).

hostapd interface initialization is now completed in a callback, if
needed, i.e., he_features channel/hw_mode selection can use as much time
as needed. This can also help with radar detection in the future.