iio: ade7753: avoid uninitialized data

The ade7753_spi_read_reg_16() will either successfully read a value
from SPI, or return a failure code without delivering data. However,
the ade7753_stop_device() and ade7753_reset() functions use the returned
data without checking for an error condition first. Gcc detects this
as a possible bug and warns about it:

drivers/staging/iio/meter/ade7753.c: In function 'ade7753_remove':
drivers/staging/iio/meter/ade7753.c:348:6: error: 'val' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  val |= BIT(4);  /* AD converters can be turned off */
      ^
drivers/staging/iio/meter/ade7753.c:345:6: note: 'val' was declared here
  u16 val;
      ^
drivers/staging/iio/meter/ade7753.c: In function 'ade7753_probe':
drivers/staging/iio/meter/ade7753.c:222:6: error: 'val' may be used uninitialized in this function [-Werror=maybe-uninitialized]

In both cases, we can avoids the warning by checking the return code
before using the data.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/staging/iio/meter/ade7753.c

@@ -217,8 +217,12 @@ static ssize_t ade7753_write_16bit(struct device *dev,
 static int ade7753_reset(struct device *dev)
 {
 	u16 val;
+	int ret;
+
+	ret = ade7753_spi_read_reg_16(dev, ADE7753_MODE, &val);
+	if (ret)
+		return ret;
 
-	ade7753_spi_read_reg_16(dev, ADE7753_MODE, &val);
 	val |= BIT(6); /* Software Chip Reset */
 
 	return ade7753_spi_write_reg_16(dev, ADE7753_MODE, val);
@@ -343,8 +347,12 @@ static int ade7753_set_irq(struct device *dev, bool enable)
 static int ade7753_stop_device(struct device *dev)
 {
 	u16 val;
+	int ret;
+
+	ret = ade7753_spi_read_reg_16(dev, ADE7753_MODE, &val);
+	if (ret)
+		return ret;
 
-	ade7753_spi_read_reg_16(dev, ADE7753_MODE, &val);
 	val |= BIT(4);  /* AD converters can be turned off */
 
 	return ade7753_spi_write_reg_16(dev, ADE7753_MODE, val);