net: tcp: Avoid new packets from causing killing the pending send_timer

When there was no room to transmit the a next packet to be transmitted,
the -ENOBUFS could cause the retransmission to fail.

Secondly the conn->unacked_len can be set to 0 in the retransmission
process, causing the subscribtion to the transmit timer to fail. Use the
variable send_data_total instead.

Make sure that is the send_data buffer becomes empty the send_timer is
cancelled, but make sure any pending data still keeps on being transmitted.

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>