iwlwifi: mvm: clean up HE radiotap RU allocation parsing

Split the code out into a separate routine, and move that to be
called inside the previously introduced iwl_mvm_decode_he_phy_data()
function.

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