LinuxKPI: 802.11: implement some *eleme/ie* lookup functions

Implement cfg80211_find_elem(), ieee80211_bss_get_elem(),
ieee80211_bss_get_ie(), and cfg80211_find_vendor_ie() with the last
one having a short cut always also checking oui_type in the pattern.

Sponsored by: The FreeBSD Foundation
MFC after: 10 days