iwlwifi: mvm: remove unnecessary overload variable

This is equivalent to checking he_phy_data != HE_PHY_DATA_INVAL,
which is already done in a number of places, so remove the extra
'overload' variable entirely.

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