staging: iio: resolver: ad2s1210: fix ad2s1210_show_fault

When reading the fault attribute, an empty string was printed if the
fault register value was non-zero.

This is fixed by checking that the return value is less than zero
instead of not zero.

Also always print two hex digits while we are touching this line.
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20230921144400.62380-4-dlechner@baylibre.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/staging/iio/resolver/ad2s1210.c

@@ -393,7 +393,7 @@ static ssize_t ad2s1210_show_fault(struct device *dev,
 	ret = ad2s1210_config_read(st, AD2S1210_REG_FAULT);
 	mutex_unlock(&st->lock);
 
-	return ret ? ret : sprintf(buf, "0x%x\n", ret);
+	return ret < 0 ? ret : sprintf(buf, "0x%02x\n", ret);
 }
 
 static ssize_t ad2s1210_clear_fault(struct device *dev,