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.
This fixes enumeration issues on Windows.

Removes the former TODO in the configuration descriptor handling.
It is not needed and was masking this issue.

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;
     }
 
@@ -447,11 +454,6 @@ static void pbdrv_usb_handle_std_request(pbdrv_usb_nxt_setup_packet_t *packet) {
                     size = sizeof(pbdrv_usb_nxt_full_config);
                     pbdrv_usb_nxt_write_data(0, &pbdrv_usb_nxt_full_config,
                         MIN(size, packet->length));
-
-                    /* TODO: Why? This is not specified in the USB specs. */
-                    if (pbdrv_usb_nxt_full_config.conf_desc.wTotalLength < packet->length) {
-                        pbdrv_usb_nxt_send_null();
-                    }
                     break;
 
                 case USB_DESC_TYPE_STR: /* String or language info. */
@@ -776,8 +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 {