mac80211: rename rtap_vendor_space to rtap_space

Since all the HE data won't fit into struct ieee80211_rx_status,
we'll (have to) move that into the SKB proper. This means we'll
need to skip over more things in the future, so rename this to
remove the vendor-only notion from it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>