tools/bootconfig: Fix apply_xbc() to return zero on success

The return of apply_xbc() returns the result of the last write() call, which
is not what is expected. It should only return zero on success.

Link: https://lore.kernel.org/r/20200508093059.GF9365@kadam



Fixes: 88426044 ("tools/bootconfig: Fix resource leak in apply_xbc()")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Tested-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>