qcacmn: get length of nbuf after pushing the meta data in mesh mode

get length of nbuf and set it in tx_desc after pushing the
meta header in mesh mode

Change-Id: Ide563a339eae392c7b3f512526d02f7ec2d6c1df

dp/wifi3.0/dp_tx.c

@@ -961,6 +961,8 @@ struct dp_tx_desc_s *dp_tx_prepare_desc_single(struct dp_vdev *vdev,
 				msdu_info);
 		if (htt_hdr_size == 0)
 			goto failure;
+
+		tx_desc->length = qdf_nbuf_headlen(nbuf);
 		tx_desc->pkt_offset = align_pad + htt_hdr_size;
 		tx_desc->flags |= DP_TX_DESC_FLAG_TO_FW;
 		is_exception = 1;