mptcp: avoid error message on infinite mapping

Another left-over. Avoid flooding dmesg with useless text,
we already have a MIB for that event.

Fixes: 648ef4b8

 ("mptcp: Implement MPTCP receive path")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>