selftests: mptcp: join: no extra msg if no counter

The checksum and fail counters might not be available. Then no need to
display an extra message with missing info.

While at it, fix the indentation around, which is wrong since the same
commit.

Fixes: 47867f0a ("selftests: mptcp: join: skip check if MIB counter not supported")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>