jobmanager: fix job registration
Any call to jobmgr_register_job_type() needs to be ignored once the job
manager is initialized. It's wrong to assume that this function won't
ever be called after the jobmgr_init() because there may be several
providers doing following sequence in single provider library during its
initialization:
1. lmi_init()
2. jobmgr_register_job_type()
3. jobmgr_init()