mptcp: remove unneeded check on first subflow

Currently we explicitly check for the first subflow being
NULL in a couple of places, even if we don't need any
special actions in such scenario.

Just drop the unneeded checks, to avoid confusion.

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>