selftests: mptcp: fix error path

pm_nl_check_endpoint() currently calls an not existing helper
to mark the test as failed. Fix the wrong call.

Fixes: 03668c65

 ("selftests: mptcp: join: rework detailed report")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>