pbio/drv/usb/usb_nxt.c: Fix ZLPs on the control pipe

This is needed to correctly indicate end of descriptor when the
descriptor size is an exact multiple of the endpoint packet size.

Author: ArcaneNibble

lib/pbio/drv/usb/usb_nxt.c

@@ -291,7 +291,14 @@ static void pbdrv_usb_nxt_write_data(int endpoint, const void *ptr_, uint32_t le
         pbdrv_usb_nxt_state.tx_data[tx] = (uint8_t *)(ptr + packet_size);
         pbdrv_usb_nxt_state.tx_len[tx] = length;
     } else {
-        pbdrv_usb_nxt_state.tx_data[tx] = NULL;
+        if (length == packet_size && endpoint == 0) {
+            // If we are sending data to the control pipe, we must terminate the data
+            // with a ZLP. In order to do so, we set the data pointer to non-NULL
+            // but the length to 0. We do not want to send ZLPs on the Pybricks bulk pipe.
+            pbdrv_usb_nxt_state.tx_data[tx] = (uint8_t *)(ptr);
+        } else {
+            pbdrv_usb_nxt_state.tx_data[tx] = NULL;
+        }
         pbdrv_usb_nxt_state.tx_len[tx] = 0;
     }
 
@@ -771,8 +778,7 @@ static void pbdrv_usb_nxt_isr(void) {
             }
 
             /* and we will send the following data */
-            if (pbdrv_usb_nxt_state.tx_len[endpoint] > 0
-                && pbdrv_usb_nxt_state.tx_data[endpoint] != NULL) {
+            if (pbdrv_usb_nxt_state.tx_data[endpoint] != NULL) {
                 pbdrv_usb_nxt_write_data(endpoint, pbdrv_usb_nxt_state.tx_data[endpoint],
                     pbdrv_usb_nxt_state.tx_len[endpoint]);
             } else {