sbs-battery: Probe should try talking to the device

Turns out this driver doesn't actually try talking to the device at
probe time, so if it's incorrectly configured in the device tree or
platform data (or if the battery has been removed from the system),
then probe will succeed and every access will sit there and time out.
The end result is a possibly laggy system that thinks it has a battery
but can never read status, which isn't very useful.

Instead, just read any register (I chose status) at probe, and if that
fails, don't register the device.

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Rhyland Klein <rklein@nvidia.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>

drivers/power/sbs-battery.c

@@ -759,6 +759,16 @@ static int __devinit sbs_probe(struct i2c_client *client,
 	chip->irq = irq;
 
 skip_gpio:
+	/*
+	 * Before we register, we need to make sure we can actually talk
+	 * to the battery.
+	 */
+	rc = sbs_read_word_data(client, sbs_data[REG_STATUS].addr);
+	if (rc < 0) {
+		dev_err(&client->dev, "%s: Failed to get device status\n",
+			__func__);
+		goto exit_psupply;
+	}
 
 	rc = power_supply_register(&client->dev, &chip->power_supply);
 	if (rc) {