binder: fix an ret value override

commit 372e3147 ("binder: guarantee txn complete / errors delivered
in-order") incorrectly defined a local ret value.  This ret value will
be invalid when out of the if block

Fixes: 372e3147 ("binder: refactor binder ref inc/dec for thread safety")
Signed-off-by: Xu YiPing <xuyiping@hislicon.com>
Signed-off-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>