power: pm8921-charger: add reverse boost fix for dc_in

Reverse boosting is also seen on the DC charging path
and may cause charger removal detection problems.

Fix this by changing the reverse boost detection to only
check the active charging path. Only the active charging
path will see reverse boosting voltages because the other
path's OVP will automatically be opened (off) by the
hardware.

Change-Id: I23a48cd49d912e2bb2651159ec91d0ccfa7f5e0d
Signed-off-by: David Keitel <dkeitel@codeaurora.org>