iwlwifi: mvm: report RU offset is known

We already report the RU offset, so we'd better also
report that we know the value.

Fixes: e5721e3f ("iwlwifi: mvm: add radiotap data for HE")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>