tools/bootconfig: Fix error return code in apply_xbc()

Fix to return a negative error code from the error handling case instead
of 0, as done elsewhere in this function.

Link: https://lkml.kernel.org/r/20210508034216.2277-1-thunder.leizhen@huawei.com

Fixes: a995e6bc

 ("tools/bootconfig: Fix to check the write failure correctly")
Reported-by: Hulk Robot <hulkci@huawei.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>