iwlwifi: mvm: fix pointer reference when setting HE QAM thres

Pointer referencing when setting HE QAM thresholds (when nominal
packet padding bit is on) caused kernel crash due to bad
referencing. Fix that.

Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>