PM / devfreq: Fix warning in governor_cpufreq

The cpu_khz variable would be uninitialized when debug messages are printed
for OFFLINE CPUs. Fix that by initializing it to 0.

Change-Id: Ic707c13b1048625c0ecc0b0057af2f84f17462f6
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>