power: qpnp-bms: prevent shorting input leads for external rsense

The BMS has a feature where when the batfet is open it shorts the
IADC's input leads so that a zero current is read by BMS state
machine.
This feature is very useful when internal Rsense is used, but
may adversely affects current readings if external Rsense is used
and there are components that draw current through it.

Fix this by turning the "shorting input leads" feature off when
external Rsense is in use.

CRs-Fixed: 509620
Change-Id: I643e4df92291a787ba91f6204fc728c26dc80734
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>