staging: iio: resolver: ad2s1210: fix ad2s1210_show_fault

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.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>