bootconfig: Fix missing return check of xbc_node_compose_key function

The function `xbc_show_list should` handle the keys during the
composition. Even the errors returned by the compose function. Instead
of removing the `ret` variable, it should save the value and show the
exact error. This missing variable is causing a compilation issue also.

Link: https://lkml.kernel.org/r/163077087861.222577.12884543474750968146.stgit@devnote2

Fixes: e5efaeb8

 ("bootconfig: Support mixing a value and subkeys under a key")
Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>