sched/fair: avoid negative delta in feec()

If utilization values are updated in between calls to
compute_energy(), this can result in a negative delta value.
In such case, bail out of feec(), meaning the task will be placed
on prev_cpu. The task will have a chance to be migrated in the next
call to feec().

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>