PM / devfreq: Skip status update on uninitialized previous_freq

In case devfreq->previous_freq is still uninitialized in
devfreq_update_status(), i.e. it has value '0', the lookups in
that function fail, eventually leading to some error message:
[    3.041292] devfreq bus_dmc: Couldn't update frequency transition information.

Just skip the statup update in this situation.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>