cpufreq: interactive: Fix potential divide-by-zero operation

prev_load could be zero if no active time is registered for a CPU
within a sampling period. Fix potential divide-by-zero issue when
calculating new load percentage.

Change-Id: I8ad118f5b6b94a410ec59eb5ce939b9467e921c7
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>
Signed-off-by: Hanumath Prasad <hpprasad@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>