network: Set driver pointer to NULL back if not ready

network: Set driver pointer to NULL back if not ready

The network_probe() function will be called several times until it
creates successfuly the service object.

In the case of the oFono plugin, the connman_device_add_network() is
called after the driver has been registered to the network. Therefore
the network->driver has been set but no device yet.
__connman_service_create_from_network() returns -EINVAL because
the device is not yet set.

The next call on network_probe() will bail out on 'if (network->driver
!= NULL) return -EALREADY;' so we have to reset it in the first round.

Fixes BMC #21705