sdcard: Use the ldst API

sdcard: Use the ldst API

The load/store API will ease further code movement.

Per the Physical Layer Simplified Spec. "3.6 Bus Protocol":

  "In the CMD line the Most Significant Bit (MSB) is transmitted
   first, the Least Significant Bit (LSB) is the last."

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>