sched: fair: Fix potential array out-of-bounds read in __find_new_capacity

To prevent reads past the size of the array, return the index of the
maximum capacity value in the capacity-states array if we can't find an
index with a capacity higher than the utilization passed in to
__find_new_capacity.

CRs-Fixed: 2115053
Change-Id: I20599134c9db4ae9fd298cab6197d2b4e866d3d8
Signed-off-by: Puja Gupta <pujag@codeaurora.org>