power: bq28400: Verify device exist

Verify that the device exist before registering battery power supply.
This is for the case that the device power-up with DC power supply
and without battery.

Change-Id: I2935d7034803eef7f6b0f283b829c50d1ce478a6
Signed-off-by: Amir Samuelov <amirs@codeaurora.org>