stmmac: DMA threshold mode or SF mode can be different among multiple device instance

- In tx_hard_error_bump_tc interrupt, tc should be bumped only when current
device instance is in DMA threshold mode. Check per device xstats.threshold
other than global tc.

- Set per device xstats.threshold to SF_DMA_MODE when current device
instance is set to SF mode.

v2-changes:
- fix ident style

Signed-off-by: Sonic Zhang <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

···
 		 *    that needs to not insert csum in the TDES.
 		 */
 		priv->hw->dma->dma_mode(priv->ioaddr, SF_DMA_MODE, SF_DMA_MODE);
-		tc = SF_DMA_MODE;
+		priv->xstats.threshold = SF_DMA_MODE;
 	} else
 		priv->hw->dma->dma_mode(priv->ioaddr, tc, SF_DMA_MODE);
 }
···
 	}
 	if (unlikely(status & tx_hard_error_bump_tc)) {
 		/* Try to bump up the dma threshold on this failure */
-		if (unlikely(tc != SF_DMA_MODE) && (tc <= 256)) {
+		if (unlikely(priv->xstats.threshold != SF_DMA_MODE) &&
+		    (tc <= 256)) {
 			tc += 64;
 			if (priv->plat->force_thresh_dma_mode)
 				priv->hw->dma->dma_mode(priv->ioaddr, tc, tc);