net: config: Fix improper condition in net_config_init

Currently, there is a case for net_config_init function that for
timeout==0 and when iface is already up, the ip setup is not proceed
and the error message "Timeout while waiting network..." is logged.

This commit fixes it.

Signed-off-by: Lukasz Maciejonczyk <lukasz.maciejonczyk@nordicsemi.no>