power: smb349-charger: Add a check to detect if the charger is present

There is a possibility that the device is instantiated in the device
tree but is not present on the board. Add a dummy read to check if
this is case and bail out.

Change-Id: Ie3f6ce6a88d2b4bd5348e5e8f8c1cb650e15dc77
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>