sched/walt: eval_need code and trace cleanup

Improve the naming and types of variables in eval_need,
reduce computation and adjust the tracepoint to reflect
the new names of variables.

Change-Id: Ifc19bc802bbc8382577b0e4ed4b4c9be77ffe99e
Signed-off-by: Stephen Dickey <dickey@codeaurora.org>
Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org>