Add the missing platform_device_unregister() before return
from virt_platform_init() in the error handling case.

Fixes: 05d51e42 ("m68k: Introduce a virtual m68k machine")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Link: https://lore.kernel.org/r/20220628084903.3147123-1-yangyingliang@huawei.comSigned-off-by: Geert Uytterhoeven <geert@linux-m68k.org>