net: tcp: Retrigger the send work in case of loopback

In case a loopback or own address is used in TCP connection, the TCP
stack delegates the acatual data send to a workqueue. This is fine,
however it could lead to some aritificial delays in case a lot of data
is being sent before the workqueue has a chance to execute queued work
items. In such case, we only sent a single packet, when many could've
already been queued.

Fix this, by resubmitting the queue in case a local address is used, and
there's still more packets pending for send.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>