power: pm8921-bms: use direct charger api instead of power supply api

The power supply api reports usb as missing if a usb wall charger
is plugged in - this is expected as a usb wall charger should be
treated as a dc charger - not a usb charger.

The bms needs to know if anything is connected to the usb port, it
does not care whether it is a usb wall charger or pc usb charger.

Use a direct api exposed by the charger that reports the USBIN_VALID
bit.

Change-Id: Ide05f7da1b14ad3b288633e887611dd5fa79276a
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>