Staging: iio: signedness bug

i2c_smbus_read_byte_data() returns an s32 type.  We need to change
"rate" to signed for the error handling to work.

Also I changed it to propogate the error code instead of just returning
-EINVAL.  Other error codes could be -EAGAIN for example.
Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/iio/magnetometer/hmc5843.c

@@ -389,11 +389,11 @@ static ssize_t show_sampling_frequency(struct device *dev,
 	struct iio_dev *indio_dev = dev_get_drvdata(dev);
 	struct i2c_client *client = to_i2c_client(indio_dev->dev.parent);
 	struct iio_dev_attr *this_attr = to_iio_dev_attr(attr);
-	u32 rate;
+	s32 rate;
 
 	rate = i2c_smbus_read_byte_data(client,  this_attr->address);
 	if (rate < 0)
-		return -EINVAL;
+		return rate;
 	rate = (rate & RATE_BITMASK) >> RATE_OFFSET;
 	return sprintf(buf, "%s\n", regval_to_samp_freq[rate]);
 }