pstore/ram: remove unnecessary ramoops_unregister_dummy()

Remove unnecessary ramoops_unregister_dummy() if ramoops
platform device register failed.

Signed-off-by: chenqiwu <chenqiwu@xiaomi.com>
Link: https://lore.kernel.org/r/1581068800-13817-2-git-send-email-qiwuchen55@gmail.com


Signed-off-by: Kees Cook <keescook@chromium.org>