net: tcp: Do not capture tx_sem during retransmission

As the stack can safely keep allocating data during retransmission mode
there is no need to take the tx_sem during retransmission any more.

Data stored in the send_data buffer will be transmitted upon the ack of
the data for which an ack is pending. This the application being fully
stalled when the TCP connection enters retransmission mode.

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