mmc: rtsx_usb_sdmmc: Re-work the code in sd_set_power_mode()

It's only at MMC_POWER_OFF and at MMC_POWER_UP when some operations must be
carried out in sd_set_power_mode(). The code is a bit obfuscated in this
regards. Let's convert it into a switch-case-clause to make this clear.

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/20250610111633.504366-5-ulf.hansson@linaro.org