net: tcp: Avoid spurious seqno decrements

There were decrements of TCP sequence numbers, inherited from FNET
stack implementation, as was used as an initial base. RFC793 does
not specify conditions for decrementing sequence numbers, so such
decrements are an artifact of FNET implementation. In Zephyr code,
we had to compensate for these decrements by extra increments
(including an increment-by-2). So, remove decrements and associated
extra increments to simplify the code.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>