wifi: simplelink: use NET_SOCKETS_OFFLOAD_PRIORITY

Use NET_SOCKETS_OFFLOAD_PRIORITY instead of
NET_SOCKETS_OFFLOAD_PRIORITY, so that by default offloaded sockets will
be used instead of native sockets.

Addiitonally this allows to select relative priority of offloaded TLS
versus native TLS when used together with NET_SOCKETS_TLS_PRIORITY.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>