eth: yt6801: donot assigning value of variable of type to itself

Fix follow compile errors with clang-19:

drivers/net/ethernet/motorcomm/yt6801/fuxi-gmac-desc.c:478:14: error: explicitly assigning value of variable of type 'int' to itself [-Werror,-Wself-assign]
  478 |         start_index = start_index;
      |         ~~~~~~~~~~~ ^ ~~~~~~~~~~~
1 error generated.

Signed-off-by: WangYuli <[email protected]>

drivers/net/ethernet/motorcomm/yt6801/fuxi-gmac-desc.c

@@ -475,7 +475,7 @@ static void fxgmac_tx_desc_init_channel(struct fxgmac_channel *channel)
 	struct fxgmac_desc_data *desc_data;
 	int start_index = ring->cur;
 	unsigned int i;
-	start_index = start_index;
+
 	/* Initialize all descriptors */
 	for (i = 0; i < ring->dma_desc_count; i++) {
 		desc_data = FXGMAC_GET_DESC_DATA(ring, i);