power: smb135x-charger: add support to always disable charging

Currently, SMB will disable charging if qcom,charging-disabled is present.
However, user could still enable charging through sysfs which might result
in crash if battery is not present on that device.

Hence update the driver so that always disable charging if
qcom,charging-disabled is present.

Change-Id: I827601cbdec31bc05408a3e1a3909358149a58a4
CRs-fixed: 661326
Signed-off-by: Ke Liu <keliu@codeaurora.org>