net: tcp: Fix the polling implementation with corrected window filling

The window full computation was corrected to use the send_total instead
of the unacked_len. This conflicted with the new polling implementation
due to the moment when these values are changed.

Move taking the tx_sem outside of tcp_send_queued_data to handle the
-ENOBUF situation properly in case called from net_tcp_queue_data.
net_tcp_queue_data removes data from the send_data in case the
transmission failed with -ENOBUF. This cause the buffer to be not full
any more.

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