sdcard: simplify SEND_IF_COND (CMD8)

replace switch(single case) -> if()

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 20180215221325.7611-16-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>