iio: light: bu27034: Fix scale format

The driver is expecting accuracy of NANOs for intensity scale in
raw_write. The IIO core is however defaulting to MICROs. This leads the
raw-write of smallest scales to never succeed as correct selector(s) are
not found.

Fix this by implementing the .write_raw_get_fmt callback to use NANO
accuracy for writes of IIO_CHAN_INFO_SCALE.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Fixes: e52afbd6

 ("iio: light: ROHM BU27034 Ambient Light Sensor")
Link: https://lore.kernel.org/r/5369117315cf05b88cf0ccb87373fd77190f6ca2.1686648422.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>