power: pm8921-charger: fix host mode charging state

While the phone is in usb host mode (supplying power to a usb peripheral)
a charging state is reported. The phone cannot be charging from usb while
supplying power to a usb peripheral.

The fix is to make the usb vbus power code disable charging from usb before
enabling the otg regulators. In addition, the power supply class should not
show a usb charger source present or online when charging is disabled.

CRs-Fixed: 322516
Change-Id: I2eb91f1ea70665b7753924537982b183eb3e8894
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>