mmc: rtsx_usb: Use MMC_CAP2_NO_SDIO

Instead of having to return -EINVAL when requested to send SDIO specific
commands, let's set MMC_CAP2_NO_SDIO as it completely prevents them.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Michał Pecio <michal.pecio@gmail.com>