fix timeout when firmware size is 10x packet size

When the firmware blob is exactly a multiple of 10 times the packet
size, the last checksum request causes the reply to the last program
request to be missed (there is only a response from the hub to the very
last program request). The program response is received, then the
checksum response is received. Since the saga is waiting for the
checksum response first, the program response is discarded.

Fix by breaking out of the loop when the firmware has been fully
transferred before making the checksum request.

Fixes: pybricks/support#178

src/sagas/flash-firmware.ts

@@ -280,7 +280,7 @@ function* flashFirmware(action: FlashFirmwareFlashAction): Generator {
         throw Error('Missing hub type in MaxProgramFlashSize');
     }
 
-    for (let offset = 0; offset < firmware.length; offset += maxDataSize) {
+    for (let offset = 0; ; ) {
         const payload = firmware.slice(offset, offset + maxDataSize);
         const programAction = (yield put(
             programRequest(info[0].startAddress + offset, payload.buffer),
@@ -289,6 +289,13 @@ function* flashFirmware(action: FlashFirmwareFlashAction): Generator {
 
         yield put(progress(offset, firmware.length));
 
+        // we don't want to request checksum if this is the last packet since
+        // the bootloader will send a response to the program request already.
+        offset += maxDataSize;
+        if (offset >= firmware.length) {
+            break;
+        }
+
         if (connectResult.canWriteWithoutResponse) {
             // Request checksum every 10 packets to prevent buffer overrun on
             // the hub because of sending too much data at once. The actual