net: tcp: Removed copy of conn->unacked_len

In the function tcp_send_data, the variable conn->unacked_len in copied
into a local variable pos. This value is only used in one location and
used mixed with the original conn->unacked_len.

This fix removes pos and switches to use conn->unacked_len everywhere
to reduce the chance of confusion. This does not functionally change the
code.

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