sched/fair: compute_energy() returns 'unsigned long'

compute_energy() currently returns 'long'. compute_energy()
internally returns em_cpu_energy() being an 'unsigned long',
and the result of compute_energy() is stored in an
'unsigned long'. Thus, it should also return an 'unsigned long'.

Suggested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>