power: smb135x-charger: Don't change current limits when batt absent

When battery is absent, the device is running from a charger source.
The default hardware behaviour/bootloader would have set the appropriate
amount of current to draw from the charger.
In such a case, ignore the values set by USB driver and continue to draw
current as set by hardware defaults/bootloader from the charger source.

Change-Id: I85c509ae9914fc31795cc02fb8a2ff2b59768e8d
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>