netfilter: xt_CT: fix error value in xt_ct_tg_check()

If setting event mask fails then we were returning 0 for success.
This patch updates return code to -EINVAL in case of problem.

Signed-off-by: Eric Leblond <eric@regit.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>