sandbox: cast to pointer from integer of different size

Building sandbox_defconfig on ARMv7 with HOST_32BIT=y results in:

drivers/misc/qfw_sandbox.c:51:25: warning:
cast to pointer from integer of different size [-Wint-to-pointer-cast]
   51 |         void *address = (void *)be64_to_cpu(dma->address);

Add the missing type conversion.

Fixes: 69512551 ("test: qemu: add qfw sandbox driver, dm tests, qemu tests")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>