stmmac: fix build when CONFIG_OF is enable

Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NGiuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

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

@@ -50,7 +50,7 @@ static int __devinit stmmac_probe_config_dt(struct platform_device *pdev,
 	 * once needed on other platforms.
 	 */
 	if (of_device_is_compatible(np, "st,spear600-gmac")) {
-		plat->pbl = 8;
+		plat->dma_cfg->pbl = 8;
 		plat->has_gmac = 1;
 		plat->pmt = 1;
 	}