iwlwifi: mvm: put LTF symbol size into HE radiotap

I evidently completely confused "number of LTF symbols" and "LTF size".
Radiotap was reporting the former, while I thought it was the latter,
and we really need both.

Add the LTF symbol size into the newly defined field in radiotap.

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