cpufreq: stats: hold reference on global cpufreq

As in f8ad216f37e6a7b07d5f6982a25c426c72f661f6,
cpufreq stat must grab a reference to the global
cpufreq kobject in order to use it. Otherwise,
the kobj->sd will be NULL, and trigger a BUG in
sysfs_create_file.

Change-Id: If7d1c13c6ebb1678e1cd33d46eba04a41a0a796d
Signed-off-by: Daniel Rosenberg <drosen@google.com>