mmc: rtsx_usb_sdmmc: Enable runtime PM autosuspend

Enable runtime PM autosuspend for the rtsx_usb_sdmmc driver to avoid the
device being runtime suspended and runtime resumed between each request.
Let's use a default timeout of 50ms, to be consistent with other mmc hosts.

Cc: Ritesh Raj Sarraf <rrs@researchut.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>