sdhci: use qemu_log_mask(UNIMP) instead of fprintf()

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 20180115182436.2066-8-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>