driver/ddr/fsl: Fix a typo in timing_cfg_8 calculation

wwt_bg should match rrt_bg. It was a typo in driver.

Signed-off-by: York Sun <yorksun@freescale.com>
diff --git a/drivers/ddr/fsl/ctrl_regs.c b/drivers/ddr/fsl/ctrl_regs.c
index 713c3ed..690e73d 100644
--- a/drivers/ddr/fsl/ctrl_regs.c
+++ b/drivers/ddr/fsl/ctrl_regs.c
@@ -1928,7 +1928,7 @@
 		wwt_bg = tccdl - 4;
 	} else {
 		rrt_bg = tccdl - 2;
-		wwt_bg = tccdl - 4;
+		wwt_bg = tccdl - 2;
 	}
 
 	acttoact_bg = picos_to_mclk(ctrl_num, common_dimm->trrdl_ps);