sdcard: use a more descriptive label 'unimplemented_spi_cmd'

sdcard: use a more descriptive label 'unimplemented_spi_cmd'

Suggested-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 20180215221325.7611-12-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>