net: fix net_tcp_state_str() declaration

The second 'const' is misguided, indicating that the returns pointer
value itself cannot be changed, but since pointers are passed by value
anyway this is not useful and was generating warnings with XCC.

The leading 'const' indicates that the memory pointed to is constant,
which is all we needed.

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