driver: wifi: simplelink: Register socket offload operations.

If the SimpleLink WiFi driver is configured, and socket offload
enabled, this revectors the Zephyr BSD socket APIs to the SimpleLink
WiFi host driver BSD socket APIs, providing a
direct offload of the TCP/IP stack to the CC3220SF network
coprocessor.

Fixes #3706

Signed-off-by: Gil Pitney <gil.pitney@linaro.org>