mmc: dw_mmc: fixed a wrong UHS_REG 16 bit clear

In the legacy code, driver clear not only UHS_REG 16 bit also 0-15bit.
If we use UHS-1 mode spec card like SDR50, SDR104. UHS_REG 0-15 should
be set by 1 according to slot id. In this case, legacy code can cause
problems.

In particular, UHS_REG consists of DDR_REG[31:16] and VOLT_REG[15:0].
Before adjusting this patch, bit[15:0] is always cleared.

Signed-off-by: Hyeonsu Kim <hyeonsu.kim@samsung.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>