Fix erroneous reference to temperature in time calculations

Travis is having problems at:  https://github.com/MarlinFirmware/Marlin/pull/8560   

So...   doing a direct commit to get things correct.   Bugfix_v1.1.x does not have this bug.  Nothing needs to be done to that branch.

Marlin/src/module/temperature.cpp

@@ -248,7 +248,7 @@ uint8_t Temperature::soft_pwm_amount[HOTENDS],
       ;
       const int8_t watch_temp_period =
         #if ENABLED(THERMAL_PROTECTION_BED) && ENABLED(PIDTEMPBED) && ENABLED(THERMAL_PROTECTION_HOTENDS) && ENABLED(PIDTEMP)
-          hotend < 0 ? temp - THERMAL_PROTECTION_BED_PERIOD : THERMAL_PROTECTION_PERIOD
+          hotend < 0 ? THERMAL_PROTECTION_BED_PERIOD : THERMAL_PROTECTION_PERIOD
         #elif ENABLED(THERMAL_PROTECTION_BED) && ENABLED(PIDTEMPBED)
           THERMAL_PROTECTION_BED_PERIOD
         #else