sdcard: reorder SDState struct members

sdcard: reorder SDState struct members

place card registers first, this will ease further code movements.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 20180215220540.6556-2-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>