power: pm8921-charger: do not report usb present while in otg

When a usb slave device is being powered by the pmic, the charger
sees a valid voltage on the usb_in line incorrectly reporting that
a valid charger source is present.

It is expected that when a slave device is present the charger is
asked to not draw any current from usb. Use this setting as a flag
to indicate if a valid charger source is present.

Also clean up the implementation by splitting the get property function
in to two - one for usb charger and other for the dc charger.

CRs-Fixed: 339000
Change-Id: I5665f7b2364c61a6f618f4725facf32b209e0249
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>