AP: Do not use struct ieee80211_mgmt::u.probe_req

AP: Do not use struct ieee80211_mgmt::u.probe_req

This struct in the union is empty, but the design of using a zero-length
u8 array here is not fully compatible with C++ and can result in
undesired compiler warnings. Since there are no non-IE fields in the
Probe Request frames, get the location of the variable length IEs simply
by using the pointer to the frame header and the known header length.

Signed-off-by: Jouni Malinen <j@w1.fi>