fix watchdog ping

Commit #d395107 didn't fix the whole issue, we also need to subtract the
sleeptime now instead of adding it. Otherwise we'd always ping the
watchdog sleeptime seconds too late.
Thanks to github user @susanwl for spotting this.

src/drivers.cpp

@@ -144,7 +144,7 @@ void TpFanDriver::ping_watchdog_and_depulse(const Level *level)
 		std::this_thread::sleep_for(depulse_);
 		set_speed(level);
 	}
-	else if (last_watchdog_ping_ + watchdog_ + sleeptime <= std::chrono::system_clock::now())
+	else if (last_watchdog_ping_ + watchdog_ - sleeptime <= std::chrono::system_clock::now())
 		set_speed(level);
 }