FIS: repare incorrect return value with ramdisk handling

Microblaze and PowerPC use boot_get_ramdisk for loading
ramdisk to memory with checking return value.
Return 0 means success. Return 1 means failed.
Here is correspond part of code from bootm.c which check
return code.

ret = boot_get_ramdisk (argc, argv, images, IH_ARCH_PPC,
		&rd_data_start, &rd_data_end);
if (ret)
	goto error;

Signed-off-by: Michal Simek <monstr@monstr.eu>