mmc: dw_mmc: fix misleading comment in dw_mci_rk3288_set_ios

DDR52 with 8-bit mode should be handled in a different way when
requesting ciu_clk. However DDR50 is used for SDMMC/SDIO and
could never be possible with 8-bit mode. It's trival but misleading.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>