iwlwifi: mvm: add LDPC-XSYM to HE radiotap data

Add information about the LDCP extra symbol segment to the HE
data when applicable (not for trigger-based PPDUs).

While at it, clean up the code for UL/DL a bit.

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