rpmsg: fix rpmsg_virtio_get_tx_buffer() no idx return in host side

In rpmsg host side, the tx buffer index is not assigned when this buffer
is obtained from the reclaimer list, this commit fix this issue.

Signed-off-by: Guiding Li <liguiding1@xiaomi.com>
Signed-off-by: Bowen Wang <wangbowen6@xiaomi.com>

lib/rpmsg/rpmsg_virtio.c

@@ -180,14 +180,15 @@ static void *rpmsg_virtio_get_tx_buffer(struct rpmsg_virtio_device *rvdev,
 		r_desc = metal_container_of(node, struct vbuff_reclaimer_t, node);
 		metal_list_del(node);
 		data = r_desc;
+		*idx = r_desc->idx;
 
 #ifndef VIRTIO_DEVICE_ONLY
-		if (role == RPMSG_HOST)
+		if (role == RPMSG_HOST) {
 			*len = rvdev->config.h2r_buf_size;
+		}
 #endif /*!VIRTIO_DEVICE_ONLY*/
 #ifndef VIRTIO_DRIVER_ONLY
 		if (role == RPMSG_REMOTE) {
-			*idx = r_desc->idx;
 			*len = virtqueue_get_buffer_length(rvdev->svq, *idx);
 		}
 #endif /*!VIRTIO_DRIVER_ONLY*/