iwlwifi: mvm: decode HE information for MU (without ext info)

When the info type is MU, we still have the data from the TSF
overload words, so should decode that. When it's MU_EXT_INFO
we additionally have the SIG-B common 0/1/2 fields.

Also document the validity depending on the info type and fix
the name of the regular TB PPDU info type accordingly.

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