iwlwifi: mvm: declare support for VHT BF info in radiotap

The driver knows whether an rx frame was beamformed and marks
it in the radiotap VHT flags. However it should also declare
that it knows to extract this info otherwise this gets discarded
by sniffers like Wireshark.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>