sched: fair: Fix load balancing for big tasks

cpu_capacity() returns maximum capacity when WALT is disabled, hence
we couldn't take advantage of CAF's optimization
Return CPU's original capacity instead to make it usable

Change-Id: I524f9f1872f038c0b77ba404b1caf0ce75321dd8
Signed-off-by: Diep Quynh <remilia.1505@gmail.com>
Signed-off-by: Alexander Winkowski <dereference23@outlook.com>