sdhci: fix a NULL pointer dereference due to uninitialized AddresSpace object

missed in 60765b6ceeb4.

  Thread 1 "qemu-system-aarch64" received signal SIGSEGV, Segmentation fault.
  address_space_init (as=0x0, root=0x55555726e410, name=name@entry=0x555555e3f0a7 "sdhci-dma") at memory.c:3050
  3050	    as->root = root;
  (gdb) bt
  #0  address_space_init (as=0x0, root=0x55555726e410, name=name@entry=0x555555e3f0a7 "sdhci-dma") at memory.c:3050
  #1  0x0000555555af62c3 in sdhci_sysbus_realize (dev=<optimized out>, errp=0x7fff7f931150) at hw/sd/sdhci.c:1564
  #2  0x00005555558b25e5 in zynqmp_sdhci_realize (dev=0x555557051520, errp=0x7fff7f931150) at hw/sd/zynqmp-sdhci.c:151
  #3  0x0000555555a2e7f3 in device_set_realized (obj=0x555557051520, value=<optimized out>, errp=0x7fff7f931270) at hw/core/qdev.c:966
  #4  0x0000555555ba3f74 in property_set_bool (obj=0x555557051520, v=<optimized out>, name=<optimized out>, opaque=0x555556e04a20,
      errp=0x7fff7f931270) at qom/object.c:1906
  #5  0x0000555555ba51f4 in object_property_set (obj=obj@entry=0x555557051520, v=v@entry=0x5555576dbd60,
      name=name@entry=0x555555dd6306 "realized", errp=errp@entry=0x7fff7f931270) at qom/object.c:1102

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180123132051.24448-1-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
