mmc: rtsx_usb_sdmmc: Convert sd_set_power_mode() into void

The sdmmc_set_ios() is the only caller of sd_set_power_mode() and it
ignores the return code. Let's therefore convert sd_set_power_mode() into a
void function instead.

Reviewed-by: Avri Altman <avri.altman@sandisk.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Ricky Wu <ricky_wu@realtek.com>
Link: https://lore.kernel.org/r/20250610111633.504366-4-ulf.hansson@linaro.org