power: pm8921-charger: ignore overvoltage/undervoltage interrupts

The pmic hardware fsm guarantees that if overvoltage and undervoltage
interrupts are high, the valid interrupts are always low. The software
need not check for overvoltage/undervoltage conditions additionally.

Change the driver to only rely on usb/dc valid interrupts.

CRs-Fixed: 310526
Change-Id: I25a1d9126384c0a235aee369be0e1b972e94a8c9
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>