iwlwifi: mvm: move HE-MU LTF_NUM parsing to he_phy_data parsing

This code gets shorter if it doesn't have to check all the
conditions, so move it to an appropriate place that has all
of them validated already.

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