iio: adc: handle unknow of_device_id data

If we get an unknown 'childmode' value, a number of variables are not
initialized properly:

drivers/iio/adc/rcar-gyroadc.c: In function 'rcar_gyroadc_probe':
drivers/iio/adc/rcar-gyroadc.c:390:5: error: 'num_channels' may be used uninitialized in this function [-Werror=maybe-uninitialized]
drivers/iio/adc/rcar-gyroadc.c:426:22: error: 'sample_width' may be used uninitialized in this function [-Werror=maybe-uninitialized]
drivers/iio/adc/rcar-gyroadc.c:428:23: error: 'channels' may be used uninitialized in this function [-Werror=maybe-uninitialized]

The driver is currently correct, but handling this properly is more robust
for possible modifications.

There is also a false-positive warning about adcmode being possibly uninitialized,
but that cannot happen as we also check the 'first' flag:

drivers/iio/adc/rcar-gyroadc.c:398:26: error: 'adcmode' may be used uninitialized in this function [-Werror=maybe-uninitialized]

This adds an initialization for 'adcmode' and bails out for any unknown childmode.

Fixes: 059c53b ("iio: adc: Add Renesas GyroADC driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/adc/rcar-gyroadc.c

@@ -336,7 +336,7 @@ static int rcar_gyroadc_parse_subdevs(struct iio_dev *indio_dev)
 	struct device_node *child;
 	struct regulator *vref;
 	unsigned int reg;
-	unsigned int adcmode, childmode;
+	unsigned int adcmode = -1, childmode;
 	unsigned int sample_width;
 	unsigned int num_channels;
 	int ret, first = 1;
@@ -366,6 +366,8 @@ static int rcar_gyroadc_parse_subdevs(struct iio_dev *indio_dev)
 			channels = rcar_gyroadc_iio_channels_3;
 			num_channels = ARRAY_SIZE(rcar_gyroadc_iio_channels_3);
 			break;
+		default:
+			return -EINVAL;
 		}
 
 		/*