driver: wifi: simplelink: Fix socket offload after s_addr cleanup

The commit 85925018
("net: Remove s_addr/s6_addr defines from in_addr/in6_addr")

did a (welcomed) removal of a #define s_addr for which the SimpleLink
SDK was checking, breaking the socket_offload driver build.

This patch fixes the driver to work with this new, better
s_addr symbol definition.

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