iwlwifi: mvm: fix an overflow in iwl_mvm_get_signal_strength

The idea here is to translate a value of 0 received from
the firmware to the lowest rssi figure. As rx_status->chain_signal
is a signed byte the lowest possible value is -128 and not -256.
-256 was causing 0 to get stored in the signed byte.
Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>

drivers/net/wireless/iwlwifi/mvm/rx.c

@@ -149,13 +149,13 @@ static void iwl_mvm_get_signal_strength(struct iwl_mvm *mvm,
 	    le32_to_cpu(phy_info->non_cfg_phy[IWL_RX_INFO_ENERGY_ANT_ABC_IDX]);
 	energy_a = (val & IWL_RX_INFO_ENERGY_ANT_A_MSK) >>
 						IWL_RX_INFO_ENERGY_ANT_A_POS;
-	energy_a = energy_a ? -energy_a : -256;
+	energy_a = energy_a ? -energy_a : S8_MIN;
 	energy_b = (val & IWL_RX_INFO_ENERGY_ANT_B_MSK) >>
 						IWL_RX_INFO_ENERGY_ANT_B_POS;
-	energy_b = energy_b ? -energy_b : -256;
+	energy_b = energy_b ? -energy_b : S8_MIN;
 	energy_c = (val & IWL_RX_INFO_ENERGY_ANT_C_MSK) >>
 						IWL_RX_INFO_ENERGY_ANT_C_POS;
-	energy_c = energy_c ? -energy_c : -256;
+	energy_c = energy_c ? -energy_c : S8_MIN;
 	max_energy = max(energy_a, energy_b);
 	max_energy = max(max_energy, energy_c);