power: pm8921-charger: enable unplug_check for PC USB

For some platforms disabling the input voltage
regulation is not sufficent to prevent unplug events
from going unnoticed.

Fix this by making sure the unplug_check_worker
is also running when a PC USB connection with
a current limit of 500 mA is set.

Signed-off-by: David Keitel <dkeitel@codeaurora.org>
Signed-off-by: Ajay Dudani <adudani@codeaurora.org>