drivers: modem: sim7080: Fix net API use

In b5588ed684365eedfb55822804af3d0c3cd944ab, and after in
e19d78e6070c4c3a2d315219d38ea66a100ec161 the mayority of the
Zephyr modem drivers were changed to use the Zephyr native
net_ prefixed types, but a few were missing.

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>