Some boards have SD card connectors where the power rail cannot be switched
off by the driver. If the card has not been power cycled, it may still be
using 1.8V signaling after a warm re-boot. Bootroms expecting 3.3V signaling
will fail to boot from a UHS card that continue to use 1.8V signaling.
Set initial signal voltage in mmc_power_off() to allow re-boot to function.
This fixes re-boot with UHS cards on Asus Tinker Board (Rockchip RK3288),
same issue have been seen on some Rockchip RK3399 boards.
Backport from https://lore.kernel.org/linux-rockchip/AM3PR03MB09664161A7FA2BD68B2800A7AC620@AM3PR03MB0966.eurprd03.prod.outlook.com/
Signed-off-by: Tianling Shen <cnsztl@gmail.com>
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