sched: use proper typedef for cpumask

commit 55b8e041e6b3 ("sched: take into account of limited CPU min
and max frequencies") has introduced a typedef bug and is fixed
in this change.

CRs-Fixed: 1006303
Change-Id: I6f75d4d0179a79544b4180a3ba155916c861edb5
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>