netfilter: conntrack: clean up returns in nf_conntrack_log_invalid_sysctl()

Smatch complains that these look like error paths with missing error
codes, especially the one where we return if nf_log_is_registered() is
true:

    net/netfilter/nf_conntrack_standalone.c:575 nf_conntrack_log_invalid_sysctl()
    warn: missing error code? 'ret'

In fact, all these return zero deliberately.  Change them to return a
literal instead which helps readability as well as silencing the warning.

Fixes: e89a6804 ("netfilter: load nf_log_syslog on enabling nf_conntrack_log_invalid")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Lance Yang <lance.yang@linux.dev>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>