net: socket_mgmt: Fix for coverity CID 203468

Let's use IS_ENABLED() instead of the #ifdef to get clearer code. ret
variable is used only at one place, no need for the ARG_UNUSED()

Coverity-CID: 203468
Fixes #18420

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>