mmc: dw_mmc: correct outdated comment for use_dma

Since commit 3fc7eaef ("mmc: dw_mmc: Add external dma interface
support") use_dma no longer means only the data transfer mode, and
includes dma transmission channel. So make it more clear.

Signed-off-by: Ziyuan <ziyuan.biubiu@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>