mmc: sdhci: Set UHS Mode Select field for UHS SDR25 mode

Set correct UHS Mode Select field value for UHS SDR25 (50MHz) mode.

Fixes: d1c0a220 ("mmc: sdhci: Add support for HOST_CONTROL2 and setting UHS timings")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>