ofono: Check for CM attached before registering network

ofono: Check for CM attached before registering network

We need to check the ConnectionManager attached signal before
we are allowed to connect to the gprs context:

connmand[592]: plugins/ofono.c:cm_update_attached() /zte_0 Attached 0

[...]

connmand[592]: plugins/ofono.c:context_set_active() /zte_0 active 1
connmand[592]: plugins/ofono.c:set_property() /zte_0 path /zte_0/context1
org.ofono.ConnectionContext.Active

[...]

connmand[592]: plugins/ofono.c:cm_update_attached() /zte_0 Attached 1

Fixes BMC#25112