power: smb135x-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: Ia90baa4bebfad7d95c5075b439f0dd26c921daf1
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>