iio: adc: ad7280a: fix ad7280_store_balance_timer()

iio: adc: ad7280a: fix ad7280_store_balance_timer()

Use correct argument to iio_str_to_fixpoint() to parse 3 decimal places.

iio_str_to_fixpoint() has a bit of an unintuitive API where the
fract_mult parameter is the multiplier of the first decimal place as if
it was already an integer.  So to get 3 decimal places, fract_mult must
be 100 rather than 1000.

Fixes: 96ccdbc07a74 ("staging:iio:adc:ad7280a: Standardize extended ABI naming")
Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>