Pull some of the decoding of he_phy_data into a separate function so
we don't need to check over and over again if it's valid.

While at it, fix the UL/DL bit reporting to be for all but trigger-
based frames.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>