power: pm8921-charger: Start unplug check worker for DC

It could happen that the device is powered up with a dc charger.
We should start the unplug check worker when the device starts with
dc charger or else we run the risk of not detecting the dc charger
removal.

Change-Id: I32d50caf7b7a84db2654b69baafb40a441f46bf0
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>