PM / devfreq: bw_hwmon: Allow up_scale to be set to zero

Currently the lower limit for setting up_scale is 100. Change it
to 0 so that over-voting for spikes in measured bandwidth can be
disabled by setting up_scale to 0.

Change-Id: I1e5e3d51c625163b38074fb1eda26763455ea6c5
Signed-off-by: Rohit Gupta <rohgup@codeaurora.org>