drivers: wifi: esp_at: Fix net API use

In 55c49cdb8f7e93b018f908fc8356212cc0e42da8 wifi
drivers were changed to use the Zephyr native net_ prefixed
types, but some were forgotten.
Without this fix/change the code still builds as we are by now setting
CONFIG_NET_NAMESPACE_COMPAT_MODE. But when this is not set, things
fail to build.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>