nr_mac_prepare_ra_ue() is called with the scheduler being locked via
NR_SCHED_ENSURE_LOCKED(), but if there is no free RA process, it
unlocks the scheduler via NR_SCHED_UNLOCK().  This is commit fixes this.

In short, when calling nr_mac_prepare_ra_ue(), the scheduler is locked,
and should always return with the scheduler being locked.