net: tcp.h: fix some function declarations

It's never a good idea to redefine functions as macros if intended
to be unused in some configuration

- "statement with no effect" warnings
- "unused argument" warnings
- No type checking done if the macros are used

These have been redefined as empty inline functions.
Fixes compiler warnings with XCC.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>