mac80211: support reporting 0-length PSDU in radiotap

For certain sounding frames, it may be useful to report them
to userspace even though they don't have a PSDU in order to
determine the PHY parameters (e.g. VHT rate/stream config.)
Add support for this to mac80211.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Shaul Triebitz <shaul.triebitz@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>