mmc: dw_mmc: add the card write threshold for HS400 mode

Since v2.80a, dwmmc controller introduced the card write threshold for
HS400 mode. So CardThrCtl can be supported during write operation, not
only read operation.
(Note: Only use the write threshold when mode is HS400.)

To use more compatible, removed "_rd_" from function name.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>