zero-initialize ADMADescr 'dscr' in sdhci_do_adma() to avoid:

  hw/sd/sdhci.c: In function ‘sdhci_do_adma’:
  hw/sd/sdhci.c:714:29: error: ‘dscr.addr’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
             trace_sdhci_adma("link", s->admasysaddr);
                             ^
Signed-off-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: NAlistair Francis <alistair.francis@xilinx.com>
Message-id: 20180115182436.2066-9-f4bug@amsat.org
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>