net: Support DMA threshold mode in DWMAC driver

- DMA threshold mode can be selected in board config head file.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
diff --git a/drivers/net/designware.c b/drivers/net/designware.c
index 7469e59..cc01604 100644
--- a/drivers/net/designware.c
+++ b/drivers/net/designware.c
@@ -253,8 +253,13 @@
 
 	writel(FIXEDBURST | PRIORXTX_41 | DMA_PBL, &dma_p->busmode);
 
+#ifndef CONFIG_DW_MAC_FORCE_THRESHOLD_MODE
 	writel(readl(&dma_p->opmode) | FLUSHTXFIFO | STOREFORWARD,
 	       &dma_p->opmode);
+#else
+	writel(readl(&dma_p->opmode) | FLUSHTXFIFO,
+	       &dma_p->opmode);
+#endif
 
 	writel(readl(&dma_p->opmode) | RXSTART | TXSTART, &dma_p->opmode);