iwlwifi: mvm: remove channel 2 from HE radiotap if not applicable

If the bandwidth is only 20 MHz, then the second channel doesn't
exist, but the hardware reports the CRC was OK. Suppress the data
of the second channel in the HE radiotap in this case, by marking
it as not known.

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