cpufreq: interactive: hold reference on global cpufreq kobject if needed

2361be23 changed cpufreq to add the
global cpufreq kobject to sysfs on demand.

To ensure this happens, cpufreq_interactive must hold a reference on
this object on devices where it intends to use it (i.e., devices where
have_governor_per_policy() returns false).  Otherwise a parentless
kobject will be passed to sysfs_create_group() which will subsequently
BUG().

Change-Id: I7dd03956e1d3c6c3c0cc17c799882c235804ae09
Signed-off-by: Greg Hackmann <ghackmann@google.com>