tools/bootconfig: Simplify expression

It is not necessary to define the variable ret to receive
the return value of the xbc_node_compose_key() method.

Link: https://lkml.kernel.org/r/20210414134647.1870-1-zuoqilin1@163.com

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: zuoqilin <zuoqilin@yulong.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>