sched: allow select_prev_cpu_us to be set to values greater than 100us

At present sched_select_prev_cpu_us tunable is restricted to values
below 100us.  Fix this unintended restriction.

CRs-Fixed: 972237
Change-Id: I5eaf9f40468805c396328ca1022baef32acf8de0
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>