refactor: move max airtime computation to macro

By that we make it reusable and also ensure the integer division is
executed correctly (it was correct before, but just because the mult and
div operator are left associating).

Author: fmoessbauer

src/Dispatcher.cpp

@@ -11,6 +11,8 @@ namespace mesh {
 #define MAX_RX_DELAY_MILLIS        32000  // 32 seconds
 #define MIN_TX_BUDGET_RESERVE_MS   100    // min budget (ms) required before allowing next TX
 #define MIN_TX_BUDGET_AIRTIME_DIV  2      // require at least 1/N of estimated airtime as budget before TX
+/// add some time to get the max allowed airtime for the est. airtime
+#define MAX_TX_AIRTIME_FOR_EST(est_airtime) (((est_airtime) * 3) / 2)
 
 #ifndef NOISE_FLOOR_CALIB_INTERVAL
   #define NOISE_FLOOR_CALIB_INTERVAL   2000     // 2 seconds
@@ -325,7 +327,7 @@ void Dispatcher::checkSend() {
     } else {
       memcpy(&raw[len], outbound->payload, outbound->payload_len); len += outbound->payload_len;
 
-      uint32_t max_airtime = _radio->getEstAirtimeFor(len)*3/2;
+      uint32_t max_airtime = MAX_TX_AIRTIME_FOR_EST(_radio->getEstAirtimeFor(len));
       outbound_start = _ms->getMillis();
       bool success = _radio->startSendRaw(raw, len);
       if (!success) {