net: offload: When NET_OFFLOAD is enabled bypass some net_tcp functions

* The issue is found in supporting offload module esp8266
* For device like esp8266, it's responsible for tcp/udp handling,
  no need of net_tcp related functions
* This commit is only tested for esp8266, no gurantee for other
  modules

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>