smb138x: use chg src bit to disable parallel charger

smb1355 doesn't have any facility to suspend input. One has to
disable charging using the chg_en command bit.

So, when parallel charging needs to be disabled, configure
the charger to ignore the chg_en input pin and instead use the
command register. The command register is always programmed to
disable charging.

When parallel charging needs to be enabled, configure it to
follow the chg_en input pin.

This will work for both smb1355 and smb1381 parallel chargers.

Change-Id: I50294f40927641c2e0a7c7e4e7d263592086d3a9
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>