power: pm8921-bms: use restart reason for usb detection

When the charger is not initialized it returns -EINVAL when asked
whether usb charger is plugged in. Use the restart reason to check
if we started with a charger insertion.

Change-Id: I2e8fcb4a15af3a66331a921d378bfd5d2dfe4533
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>