devfreq: Fix signedness of percentages array

It is possible that this array may contain a negative
value and if so, it is corrected using bounds checking.
However, bounds are not properly applied because the negative
value is seen as a very high unsigned integer.

Change-Id: I546d1ecacace185427d6fbf78e95bf8cb67f92c9
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>